LF clock compiler warning when compiling for arduino:mbed_nano:nano33ble (v4.2.1)

```
#warning No configuration for LF clock source. Xtal source will be used as a default configuration.
```

I have always compiled with `-Werror`. I have not changed my source code. I have only upgraded my Arduino CLI from 0.34.2 to 1.1.1, and this `#warning` has now appeared.

```
In file included from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/config/sdk_config.h:11967:0,
/home/blues/.arduino15/packages/arduino/tools/arm-none-eabi-gcc/7-2017q4/bin/arm-none-eabi-g++ -c -Wall -Wextra -g3 -nostdlib @/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/variants/ARDUINO_NANO33BLE/defines.txt @/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/variants/ARDUINO_NANO33BLE/cxxflags.txt -DARDUINO_ARCH_NRF52840 -MMD -mcpu=cortex-m4 -mfloat-abi=softfp -mfpu=fpv4-sp-d16 -DARDUINO=10607 -DARDUINO_ARDUINO_NANO33BLE -DARDUINO_ARCH_MBED_NANO -DARDUINO_ARCH_MBED -DARDUINO_LIBRARY_DISCOVERY_PHASE=0 -I/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino -I/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/variants/ARDUINO_NANO33BLE -Wno-unused-parameter -Werror -I/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/api/deprecated -I/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/api/deprecated-avr-comp -iprefix/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino @/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/variants/ARDUINO_NANO33BLE/includes.txt /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/api/CanMsg.cpp -o /home/blues/.cache/arduino/sketches/CAC5CD9D43929367233EF06CE5385189/core/api/CanMsg.cpp.o
from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_SDK_15_0/integration/nrfx/nrfx_config.h:45,
from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_SDK_15_0/modules/nrfx/nrfx.h:44,
from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_SDK_15_0/modules/nrfx/hal/nrf_gpio.h:44,
from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/TARGET_ARDUINO_NANO33BLE/PinNames.h:23,
from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/objects.h:45,
from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/TARGET_ARDUINO_NANO33BLE/device.h:37,
from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/hal/include/hal/trng_api.h:24,
from /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/WMath.cpp:26:
/home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/cores/arduino/mbed/targets/TARGET_NORDIC/TARGET_NRF5x/nrf5x_lf_clk_helper.h:43:6: error: #warning No configuration for LF clock source. Xtal source will be used as a default configuration. [-Werror=cpp]
#warning No configuration for LF clock source. Xtal source will be used as a default configuration.
^~~~~~~
cc1plus: all warnings being treated as errors

Using library Blues Wireless Notecard at version 1.6.3 in folder: /home/blues/Arduino/libraries/Blues_Wireless_Notecard
Using library Wire in folder: /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/libraries/Wire (legacy)
Error during build: exit status 1

Used library Version Path
Blues Wireless Notecard 1.6.3 /home/blues/Arduino/libraries/Blues_Wireless_Notecard
Wire /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1/libraries/Wire

Used platform Version Path
arduino:mbed_nano 4.2.1 /home/blues/.arduino15/packages/arduino/hardware/mbed_nano/4.2.1
Error: Process completed with exit code 1.
```

Reproduce the Nano 33 BLE build with Arduino CLI 1.1.1, platform arduino:mbed_nano 4.2.1, and -Werror. Start with nrf5x_lf_clk_helper.h:43 and the sdk_config.h include chain shown in the report. Done means identifying why the LF clock warning appears after the CLI upgrade and making the affected build complete without that warning becoming an error.

Written by the indexing model from the issue text.