GCS/Colibri: make board image tarnsparent backgroudn

Looks much better on bootloader.
PiOS MPU6000: add support for placing on bottom of board

Colibri mounts the sensors on the bottom, so this adds those
options to the configuration.
PiOS MPU9150: support positioning on bottom of board

This is for consistency with the new cases handled
by the MPU60X0 class of drivers.
PiOS MPU6050: add support for placing on bottom of board
Colibri: correct sensor orientation
Colibri: disable SBUS inverter when not used

This allows Spektrum or PPM to work on USART2