For use with F0 MCUs that don't have HSI48. Select the clock source
explicitly in mpconfigboard.h.
On the NUCLEO_F091RC board use HSE bypass when HSE is chosen because the
NUCLEO clock source is STLINK not a crystal.
Before this patch the UART baudrate on F0 MCUs was wrong because the
stm32lib SystemCoreClockUpdate sets SystemCoreClock to 8MHz instead of
48MHz if HSI48 is routed directly to SYSCLK.
The workaround is to use HSI48 -> PREDIV (/2) -> PLL (*2) -> SYSCLK.
Fixes issue #5049.
This new series of MCUs is similar to the L4 series with an additional
Cortex-M0 coprocessor. The firmware for the wireless stack must be managed
separately and MicroPython does not currently interface to it. Supported
features so far include: RTC, UART, USB, internal flash filesystem.
