add method to block fallback to mission

failsafe navigation modes can use a flag in mission_result to tell the
commander to not switch back to mission

src/modules/uORB/topics/mission_result.h

@@ -57,6 +57,7 @@ struct mission_result_s
 	unsigned seq_current;		/**< Sequence of the current mission item				 */
 	bool reached;				/**< true if mission has been reached					 */
 	bool finished;				/**< true if mission has been completed					 */
+	bool stay_in_failsafe;			/**< true if the commander should not switch out of the failsafe mode*/
 };
 
 /**