PX4: added conversion lib for rotation
PX4: try to startup FMUv3 with right rotations
ph2 support: Fix typo in startup rc
PX4: fixed l3gd20 rotation

based on first flight log
PX4: added comments on FMU3 rotations
AP_Compass: support 3 mags on PX4
Copter: log up to 3 mags
PX4: try to start both hmc5883 sensors

internal + external
Plane: support 3 magnetometers
Rover: support 3 magnetometers
PX4: fixed orientation of internal hmc5883 on FMUv3