Printing to stderr attempts a write into flash

A call of the form:

``` C
fprintf(stderr, "Some error: %d", 42);
```

causes a write to flash, as `stderr` is defined as `__sf_fake_stderr`, a symbol in flash.

Writes to flash through a `str` instruction do not trigger a fault, so the instruction is run without problems (of course the write is not performed).

**Note**:

> This was observed while the libC had not completed its initialization.

The following is a simplified backtrace.

```
#0 0x000141ea in __swsetup_r ()
#1 0x00013398 in _vfprintf_r ()
#2 0x000130b6 in fprintf ()
#3 0x00009c38 in core_util_assert_internal (expr=0x15ce8 "primask & 0x1", file=0x15c88 "/Users/aleang01/code/AlessandroA/uvisor-helloworld/yotta_modules/core-util/source/critical.c", line=61,
msg=0x0 <__isr_vector>)
#4 0x00009b92 in core_util_critical_section_exit () at /Users/aleang01/code/AlessandroA/uvisor-helloworld/yotta_modules/core-util/source/critical.c:61
#5 0x00008858 in mbed::util::CriticalSectionLock::~CriticalSectionLock (this=0x1fffad14, __in_chrg=)
#6 0x000148c0 in mbed::util::atomic_cas (ptr=0x1fffafd8 , expectedCurrentValue=0x1fffad3c, desiredValue=212480)
#7 0x0001479a in mbed_sbrk (size=1968)
#8 0x0000f100 in sys_alloc (m=0x1fffb868 <_gm_>, nb=432)
#9 0x00011076 in dlmalloc (bytes=428)
#10 0x0000e024 in mbed_ualloc_internal (bytes=428, traits=..., caller=0x1495b <__wrap__malloc_r+22>)
#11 0x0000e090 in mbed_ualloc (bytes=428, traits=...)
#12 0x0001495a in __wrap__malloc_r (r=0x1fffaf68 , size=428)
#13 0x00014af8 in __sfmoreglue ()
#14 0x00012fca in __sfp ()
#15 0x00012f5a in __sinit ()
#16 0x00012faa in __sfp ()
#17 0x00013028 in _fopen_r ()
#18 0x00007364 in mbed::Stream::Stream (this=0x1fffb134 , name=0x0 <__isr_vector>)
#19 0x000072aa in mbed::Serial::Serial (this=0x1fffb060 , tx=PTB17, rx=PTB16, name=0x0 <__isr_vector>)
#20 0x00006e44 in __static_initialization_and_destruction_0 (__initialize_p=1, __priority=65535)
#21 0x00006ec2 in _GLOBAL__sub_I___reserved_stack ()
#22 0x0001326e in __libc_init_array ()
#23 0x000062ee in _start ()
```

In that case, the offending write was done in `__swsetup_r`:

```
strh r3, [r4, #12]
```

with `r4 = 0x16970`, which is in flash.

In case it was confirmed that the error only occurs at initialization time (`stderr` is set correctly after that), this still needs fixing as uVisor will throw an exception on an attempt to write to flash.

@bremoran @0xc0170 @bogdanm

No repository file is named. Start by tracing stderr initialization through __libc_init_array, __sinit, __sfp, and __swsetup_r, then compare that path with mbed::Serial static initialization. Reproduce fprintf(stderr, ...) during initialization and confirm that the completed fix prevents any write to the flash-backed __sf_fake_stderr and avoids a uVisor exception.

Written by the indexing model from the issue text.