Plane: use abort landing logic with mavlink GO_AROUND cmd

- also enabled use of CMD_DO_GO_AROUND altitude param

ArduPlane/ArduPlane.cpp

@@ -836,6 +836,7 @@ void Plane::update_flight_stage(void)
             } else if (mission.get_current_nav_cmd().id == MAV_CMD_NAV_LAND) {
 
                 if ((g.land_abort_throttle_enable && channel_throttle->control_in > 95) ||
+                        auto_state.commanded_go_around ||
                         flight_stage == AP_SpdHgtControl::FLIGHT_LAND_ABORT){
                     // abort mode is sticky, it must complete while executing NAV_LAND
                     set_flight_stage(AP_SpdHgtControl::FLIGHT_LAND_ABORT);

ArduPlane/GCS_Mavlink.cpp

@@ -1384,9 +1384,17 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
             //Not allowing go around at FLIGHT_LAND_FINAL stage on purpose --
             //if plane is close to the ground a go around coudld be dangerous.
             if (plane.flight_stage == AP_SpdHgtControl::FLIGHT_LAND_APPROACH) {
-                //Just tell the autopilot we're done landing so it will 
+                // Initiate an aborted landing. This will trigger a pitch-up and
-                //proceed to the next mission item.  If there is no next mission
+                // climb-out to a safe altitude holding heading then one of the
-                //item the plane will head to home point and loiter.
+                // following actions will occur, check for in this order:
+                // - If MAV_CMD_CONTINUE_AND_CHANGE_ALT is next command in mission,
+                //      increment mission index to execute it
+                // - else if DO_LAND_START is available, jump to it
+                // - else decrement the mission index to repeat the landing approach
+
+                if (!is_zero(packet.param1)) {
+                    plane.auto_state.takeoff_altitude_rel_cm = packet.param1 * 100;
+                }
                 plane.auto_state.commanded_go_around = true;
                
                 result = MAV_RESULT_ACCEPTED;

ArduPlane/landing.cpp

@@ -16,15 +16,6 @@ bool Plane::verify_land()
     // so we don't verify command completion. Instead we use this to
     // adjust final landing parameters
 
-    // If a go around has been commanded, we are done landing.  This will send
-    // the mission to the next mission item, which presumably is a mission
-    // segment with operations to perform when a landing is called off.
-    // If there are no commands after the land waypoint mission item then
-    // the plane will proceed to loiter about its home point.
-    if (auto_state.commanded_go_around) {
-        return true;
-    }
-
     // when aborting a landing, mimic the verify_takeoff with steering hold. Once
     // the altitude has been reached, restart the landing sequence
     if (flight_stage == AP_SpdHgtControl::FLIGHT_LAND_ABORT) {