[rotorcraft] big fat attempt to clean stabilization and guidance

- No more STABILIZATION_ATTITUDE_FLOAT_x in airframe file.
- Hopefully seamlessly switch between int and float.
- Also make it possible to not use a reference at all, e.g. for passthrough
[rotorcraft] remove unused STICKS_RMAT312 hack
[rotorcraft] horizontal guidance mode switching

If swtiching to HOVER or NAV mode:
reset attitude stabilization if previous mode was not using it
[rotorcraft] fix STAB_ATTITUDE telemetry
[stm32] fix init for UART6
[stm32][uart] refactor pin configuration

UART pins on F1 boards now also defined in board files