Compiler warnings

When building with Ubuntu 19.04's arm-none-eabi-gcc (`(15:7-2018-q2-4) 7.3.1 20180622 (release) [ARM/embedded-7-branch revision 261907]`):

```
[116/324] Building CXX object ym/microbit-dal/source/CMakeFiles/microbit-dal.dir/core/MicroBitFiber.cpp.o
yotta_modules/microbit-dal/source/core/MicroBitFiber.cpp: In function 'void scheduler_init(EventModel&)':
yotta_modules/microbit-dal/source/core/MicroBitFiber.cpp:189:5: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
if (fiber_scheduler_running())
^~
yotta_modules/microbit-dal/source/core/MicroBitFiber.cpp:194:2: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
messageBus = &_messageBus;
^~~~~~~~~~
yotta_modules/microbit-dal/source/core/MicroBitFiber.cpp: In function 'int fiber_wait_for_event(uint16_t, uint16_t)':
yotta_modules/microbit-dal/source/core/MicroBitFiber.cpp:388:5: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
if(ret == MICROBIT_OK)
^~
yotta_modules/microbit-dal/source/core/MicroBitFiber.cpp:391:2: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
return ret;
^~~~~~
[122/324] Building CXX object ym/microbit-dal/source/CMakeFiles/microbit-dal.dir/core/MicroBitHeapAllocator.cpp.o
yotta_modules/microbit-dal/source/core/MicroBitHeapAllocator.cpp: In function 'void free(void*)':
yotta_modules/microbit-dal/source/core/MicroBitHeapAllocator.cpp:342:13: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
if (*cb == 0 || *cb & MICROBIT_HEAP_BLOCK_FREE)
^~
yotta_modules/microbit-dal/source/core/MicroBitHeapAllocator.cpp:345:10: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
*cb |= MICROBIT_HEAP_BLOCK_FREE;
^
[174/324] Building CXX object ym/microbit-dal/source/CMakeFiles/microbit-dal.dir/bluetooth/MicroBitIOPinService.cpp.o
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp: In member function 'void MicroBitIOPinService::onDataWritten(const GattWriteCallbackParams*)':
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:179:42: warning: array subscript is above array bounds [-Warray-bounds]
io.pin[i].getDigitalValue();
~~~~~~~~~~~~~~~~~~~~~~~~~^~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:182:41: warning: array subscript is above array bounds [-Warray-bounds]
io.pin[i].getAnalogValue();
~~~~~~~~~~~~~~~~~~~~~~~~^~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:199:41: warning: array subscript is above array bounds [-Warray-bounds]
io.pin[i].getDigitalValue();
~~~~~~~~~~~~~~~~~~~~~~~~~^~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:202:40: warning: array subscript is above array bounds [-Warray-bounds]
io.pin[i].getAnalogValue();
~~~~~~~~~~~~~~~~~~~~~~~~^~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:224:43: warning: array subscript is above array bounds [-Warray-bounds]
io.pin[pin].setAnalogValue(value);
~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:225:46: warning: array subscript is above array bounds [-Warray-bounds]
io.pin[pin].setAnalogPeriodUs(period);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:245:51: warning: array subscript is above array bounds [-Warray-bounds]
io.pin[data->pin].setDigitalValue(data->value);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:247:50: warning: array subscript is above array bounds [-Warray-bounds]
io.pin[data->pin].setAnalogValue(data->value == 255 ? 1023 : data->value << 2);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp: In member function 'void MicroBitIOPinService::updateBLEInputs(bool)':
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:136:50: warning: array subscript is above array bounds [-Warray-bounds]
value = io.pin[i].getDigitalValue();
~~~~~~~~~~~~~~~~~~~~~~~~~^~
yotta_modules/microbit-dal/source/bluetooth/MicroBitIOPinService.cpp:138:49: warning: array subscript is above array bounds [-Warray-bounds]
value = io.pin[i].getAnalogValue() >> 2;
~~~~~~~~~~~~~~~~~~~~~~~~^~
[192/324] Building C object source/CMakeFiles/microbit-micropython.dirsource/py/compile2.c.o
source/py/compile2.c: In function 'compile_const_object':
source/py/compile2.c:2550:79: warning: unused parameter 'ptop' [-Wunused-parameter]
STATIC void compile_const_object(compiler_t *comp, const byte *p, const byte *ptop) {
^~~~
[214/324] Building C object source/CMakeFiles/microbit-micropython.dirsource/lib/sam/sam.c.o
source/lib/sam/sam.c: In function 'SAMMain':
source/lib/sam/sam.c:125:5: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
if (debug)
^~
source/lib/sam/sam.c:127:2: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
InsertBreath(sam);
^~~~~~~~~~~~
source/lib/sam/sam.c:134:5: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
if (strcmp(sam_error, "OK"))
^~
source/lib/sam/sam.c:136:2: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
return 1;
^~~~~~
[249/324] Building C object source/CMakeFiles/microbit-micropython.dirsource/py/nlrthumb.c.o
source/py/nlrthumb.c: In function 'nlr_push':
source/py/nlrthumb.c:40:57: warning: unused parameter 'nlr' [-Wunused-parameter]
__attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) {
^~~
[254/324] Building C object source/CMakeFiles/microbit-micropython.dirsource/lib/sam/render.c.o
source/lib/sam/render.c: In function 'Read':
source/lib/sam/render.c:51:11: warning: comparison is always false due to limited range of data type [-Wtype-limits]
if (p > RENDER_FRAMES) {
^
source/lib/sam/render.c: In function 'Write':
source/lib/sam/render.c:71:11: warning: comparison is always false due to limited range of data type [-Wtype-limits]
if (p > RENDER_FRAMES) {
^
source/lib/sam/render.c: In function 'RenderSample':
source/lib/sam/render.c:170:5: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
if (X >= sizeof(tab48426))
^~
source/lib/sam/render.c:172:2: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
mem53 = tab48426[X];
^~~~~
[264/324] Building C object source/CMakeFiles/microbit-micropython.dirsource/py/emitinlinethumb.c.o
source/py/emitinlinethumb.c: In function 'emit_inline_thumb_end_pass':
source/py/emitinlinethumb.c:97:75: warning: unused parameter 'type_sig' [-Wunused-parameter]
STATIC void emit_inline_thumb_end_pass(emit_inline_asm_t *emit, mp_uint_t type_sig) {
^~~~~~~~
[265/324] Building CXX object source/CMakeFiles/microbit-micropython.dirsource/microbit/modaudio.cpp.o
source/microbit/modaudio.cpp: In function 'void* audio_frame_binary_op(mp_uint_t, mp_obj_t, mp_obj_t)':
source/microbit/modaudio.cpp:601:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
lhs = copy(lhs);
~~~~^~~~~~~~~~~
source/microbit/modaudio.cpp:602:5: note: here
case MP_BINARY_OP_INPLACE_ADD:
^~~~
source/microbit/modaudio.cpp:610:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
lhs = copy(lhs);
~~~~^~~~~~~~~~~
source/microbit/modaudio.cpp:611:5: note: here
case MP_BINARY_OP_INPLACE_MULTIPLY:
^~~~
[271/324] Building C object source/CMakeFiles/microbit-micropython.dirsource/py/gc.c.o
source/py/gc.c: In function 'gc_sweep':
source/py/gc.c:281:27: warning: this statement may fall through [-Wimplicit-fallthrough=]
free_tail = 1;
~~~~~~~~~~^~~
source/py/gc.c:288:13: note: here
case AT_TAIL:
^~~~
[277/324] Building C object source/CMakeFiles/microbit-micropython.dirsource/py/objset.c.o
source/py/objset.c: In function 'set_unary_op':
source/py/objset.c:465:16: warning: this statement may fall through [-Wimplicit-fallthrough=]
if (MP_OBJ_IS_TYPE(self_in, &mp_type_frozenset)) {
^
source/py/objset.c:479:9: note: here
default: return MP_OBJ_NULL; // op not supported
^~~~~~~
[321/324] Building C object source/CMakeFiles/microbit-micropython.dirsource/py/lexer.c.o
source/py/lexer.c: In function 'parse_string_literal':
source/py/lexer.c:322:32: warning: this statement may fall through [-Wimplicit-fallthrough=]
if (lex->tok_kind == MP_TOKEN_BYTES) {
^
source/py/lexer.c:328:25: note: here
case 'x':
^~~~
```
Some of these warnings are from upstream microbit and yotta and the DAL.