Rover: fix for X8R receiver on SBUS in failsafe

the X8R briefly gives PWM of 874 on all channels when the receiver is
setup as "no pulses" failsafe handling. This was interpreted as switch
position 0, which was AUTO for me.

This change rejects pulse widths below 900 for mode changes
GPS: added set_secondary() function

for preventing notify updates for 2nd GPS
Plane: set 2nd GPS as secondary
Copter: set 2nd GPS as secondary
Rover: set 2nd GPS as secondary
Flymaple: RCOUtput disable_ch() now supports disabling outputs
AntennaTracker: now supports mavlink arm and disarm commands to disable antenna servo outputs
AntennaTracker: finish adding GPS support, including using relative or absolute altitudes
AntennaTracker: use gcs[] array instead of old idiom gcs0, gcs3
AntennaTracker: add support for MANUAL and AUTO modes
AntennaTracker: initial documentation
AntennaTracker: update documentation
AntennaTracker: Improvements to yaw slewing and ballerina mode
Frame_params: Updated tilt angles for Tarot Gimbals
AP_NavEKF : Fix bug that leaves height unconstrained in static mode
AP_TECS : Add protection for airspeed runaway due to pitch errors
AP_NavEKF : Add low speed magnetometer calibration option
AP_NavEKF : Add sideslip constraint to allow plane flight without magnetometer
AP_NavEKF : update comments
AP_NavEKF : Make all compass use autodetect