AntennaTracker: added boardconfig object

this allows the safety switch to be disabled
GCS_MAVLink: XML for MAV_CMD_DO_FENCE_ENABLE and MAV_SYS_STATUS_GEOFENCE
GCS_MAVLink: Generated code from message additions for autofence.
Plane: Sends MAV_SYS_STATUS_GEOFENCE and also allows GCS to enable/disable fence.
Plane: FENCE_RET_RALLY param. Plane returns to rally point on breach.
Plane: Changes needed prior to merging with master identified by Tridge.
Plane: we need 5 points for a valid geofence

the first and last points need to be the same for it to be a closed
polygon
Plane: fixed reverting modes when FENCE_SET_RALLY is enabled

we need to keep the guided destination we are using