[stm32] Fixes for USART interrupt handler.

The interrupt handler occasionally hangs in case of noise on the line.
Fixes were two:
1) Disable IDLE line interrupt, disable LIN break interrupt in the port settings.
   (neither of them should be used for asynchronous single buffer communication).
2) In interrupt handler routine, explicitly check for the error flags, only if
   no errors occur check the received byte. Otherwise clear the USART_DR and
   log the error.

Added USART_ERRORS messages to monitor problems on USART lines.

Closes #418
[rotorcraft] silence warning in guidance
[rotorcraft] alternate buses when sending I2C_ERRORS message