Copter: fix to drift's yaw rate

The ACRO_P parameter needs to be multiplied by the roll and pitch
velocities to calculate the final yaw rate
autotest: support antenna trackers with on/off servos in SITL
AntennaTracker: support on/off servos

these servos are either moving at a constant rate, or off. To really
move them accurately we need to know about this and not use a PID.

This doesn't yet support ballerina, but could do with a bit more work.
Plane: fixed compass logging for COMPASS_PRIMARY != 0
AP_Compass: VRBRAIN corrected function definition. Add support for two compasses.