Copter: enable GCS failsafe to RTL by default

This only triggers if the user has been using the GCS's RC override
SITL: prevent wind effects at negative altitudes

this prevents crashes on takeoff with barometer noise
AntennaTracker: fixed manual control, and added baro calibration

this allows baro offsets to be compensated for before takeoff
AntennaTracker: added SERIAL_CONTROL support

allows for radio update and config over USB
AntennaTracker: don't use EEPROM home if a startup home is set in parameters
AntennaTracker: added SCAN mode

this is used to test calibrate the compass, and to test tracking PIDs