GCS_Mavlink: return MISSION_INVALID when conversion fails

Requested by MichaelO to allow better error message to be displayed to
user when the mission upload from the GCS to vehicle fails
AntennaTracker: lots of new parameters

YAW_SLEW_TIME, PITCH_SLEW_TIME, MIN_REVERSE_TIME, START_LATITUDE,
START_LONGITUDE and STARTUP_DELAY

See the parameter docs for explanations
AntennaTracker: added STOP mode

to stop tracker moving
AntennaTracker: fixed ballerina and more accurate tracking

implement new parameter options
AntennaTracker: fixed APM2 serial speed handling

need to change based on USB mux status
AP_NavEKF : Prevent large magnetic field adaptations early in flight
Merge branch 'master' of https://github.com/priseborough/ardupilot

* 'master' of https://github.com/priseborough/ardupilot:
  AP_NavEKF : Prevent large magnetic field adaptations early in flight