actuator: add support for MAV_CMD_DO_SET_ACTUATOR

Adds support for using the MAVLink command MAV_CMD_DO_SET_ACTUATOR to
update the actuator values on control group 3 aux{1, 2, 3}. A simple
deconfliction with rc_update is implemented: when a MAVLink command is
sent, RC is disabled for that channel until a major stick movement is
detected.

src/modules/mavlink/mavlink_receiver.cpp

@@ -501,8 +501,39 @@ void MavlinkReceiver::handle_message_command_both(mavlink_message_t *msg, const
 			send_ack = true;
 		}
 
-	} else {
+	} else if (cmd_mavlink.command == MAV_CMD_DO_SET_ACTUATOR) {
+		// since we're only paying attention to 3 AUX outputs, the
+		// index should be 0, otherwise ignore the message
+		if (((int) vehicle_command.param7) == 0) {
+			actuator_controls_s actuator_controls{};
+			actuator_controls.timestamp = hrt_absolute_time();
 
+			// update with existing values to avoid changing unspecified controls
+			_actuator_controls_3_sub.update(&actuator_controls);
+
+			bool updated = false;
+
+			if (PX4_ISFINITE(vehicle_command.param1)) {
+				actuator_controls.control[5] = vehicle_command.param1;
+				updated = true;
+			}
+
+			if (PX4_ISFINITE(vehicle_command.param2)) {
+				actuator_controls.control[6] = vehicle_command.param2;
+				updated = true;
+			}
+
+			if (PX4_ISFINITE(vehicle_command.param3)) {
+				actuator_controls.control[7] = vehicle_command.param3;
+				updated = true;
+			}
+
+			if (updated) {
+				_actuator_controls_pubs[3].publish(actuator_controls);
+			}
+		}
+
+	} else {
 		send_ack = false;
 
 		if (msg->sysid == mavlink_system.sysid && msg->compid == mavlink_system.compid) {

src/modules/mavlink/mavlink_receiver.h

@@ -411,6 +411,7 @@ private:
 	uORB::Subscription	_vehicle_attitude_sub{ORB_ID(vehicle_attitude)};
 	uORB::Subscription	_vehicle_local_position_sub{ORB_ID(vehicle_local_position)};
 	uORB::Subscription	_vehicle_status_sub{ORB_ID(vehicle_status)};
+	uORB::Subscription 	_actuator_controls_3_sub{ORB_ID(actuator_controls_3)};
 
 	uORB::SubscriptionInterval _parameter_update_sub{ORB_ID(parameter_update), 1_s};