Plane: use ground steering logic for final stage of auto landing

this should improve runway tracking

ArduPlane/Attitude.pde

@@ -206,12 +206,26 @@ static void stabilize_stick_mixing_fbw()
  */
 static void stabilize_yaw(float speed_scaler)
 {
-    steering_control.ground_steering = (channel_roll->control_in == 0 && fabsf(relative_altitude()) < g.ground_steer_alt);
+    if (control_mode == AUTO && flight_stage == AP_SpdHgtControl::FLIGHT_LAND_FINAL) {
+        // in land final setup for ground steering
+        steering_control.ground_steering = true;
+    } else {
+        // otherwise use ground steering when no input control and we
+        // are below the GROUND_STEER_ALT
+        steering_control.ground_steering = (channel_roll->control_in == 0 && 
+                                            fabsf(relative_altitude()) < g.ground_steer_alt);
+    }
+
 
     /*
-      first calculate steering_control.steering for a nose or tail wheel
+      first calculate steering_control.steering for a nose or tail
+      wheel.
+      We use "course hold" mode for the rudder when either in the
+      final stage of landing (when the wings are help level) or when
+      in course hold in FBWA mode (when we are below GROUND_STEER_ALT)
      */
-    if (steer_state.hold_course_cd != -1 && steering_control.ground_steering) {
+    if ((control_mode == AUTO && flight_stage == AP_SpdHgtControl::FLIGHT_LAND_FINAL) ||
+        (steer_state.hold_course_cd != -1 && steering_control.ground_steering)) {
         calc_nav_yaw_course();
     } else if (steering_control.ground_steering) {
         calc_nav_yaw_ground();