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.