[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
[makefile] fix sim build on OS X after broken from no_custom, can't seem to build the autopilot.so properly so back to -custom just for simsitl
[Makefile] improve detection of lablgtk2-gnome.*
[rotorcraft] Add forward mode for transitioning vehicles
[rotorcraft] TRANSITION_MAX_OFFSET in radians, float fixes
[Makefile] use -fPIC for all architectures
[umarim] remove warning if no mag are used with umarim
Fixed USART interrupt handler.
Added USART_ERRORS messages to monitor problems on USART lines.
Note: libopencm3 has to be updated too.
Fixed uart error messages
Fixed settings of uart port (no changes in libopencm3 needed)