STM32F4 does not require the HSI to erase or program FLASH
The HSI needs to be left on until a new clock source is chosen.
(we leave it on all the time)
1) Ensure if CONFIG_SERIAL_DMA is set that cdcacm uart_ops is initalized
with correct functions in correct slots.
This was detected only with PX4 build flags
2) C&P error from F7 would prevent CONFIG_STM32_SERIALBRK_BSDCOMPAT ifdefed
code from being included.
David Sidrane
Mark Charlebois
Don Gagne
Lorenz Meier