mission: be more intelligent about saying that we are on a mission landing

- previously the decision of being on a landing pattern was taken by just
looking at the current mission index. However, even if the current mission
index indicates a landing pattern the vehicle could be at an arbitrary location, far
from being established on the pattern.

Signed-off-by: RomanBapst <bapstroman@gmail.com>

src/modules/navigator/mission.h

@@ -255,6 +255,8 @@ private:
 
 	bool _need_takeoff{true};					/**< if true, then takeoff must be performed before going to the first waypoint (if needed) */
 
+	hrt_abstime _time_mission_deactivated{0};
+
 	enum {
 		MISSION_TYPE_NONE,
 		MISSION_TYPE_MISSION