Rover: remove some unused functions
MAVLink: added MAV_CMD_DO_SET_CAM_TRIGG_DIST
GCS_MAVLink: regenerate mavlink headers
AP_Camera: added set_trigger_distance() call
Plane: support MAV_CMD_DO_SET_CAM_TRIGG_DIST
Copter: support MAV_CMD_DO_SET_CAM_TRIGG_DIST
Rover: support MAV_CMD_DO_SET_CAM_TRIGG_DIST