mission_block: change loiter handling logic

- add ability of "heading wait" for NAV_CMD_LOITER_TIME_LIMIT
- For both LOITER_TIME and LOITER_TO_ALT in fixed-wing flight, unify logic:
--> reach position --> start loitering --> reach altitude --> start timer (if applicable)
	--> reach exit heading (if applicable) --> declare mission item reached

Signed-off-by: Silvan Fuhrer <silvan@auterion.com>

src/modules/navigator/mission_block.h

@@ -148,9 +148,7 @@ protected:
 
 	bool _waypoint_position_reached{false};
 	bool _waypoint_yaw_reached{false};
-	bool _waypoint_position_reached_previously{false};
 
-	hrt_abstime _time_first_inside_orbit{0};
 	hrt_abstime _action_start{0};
 	hrt_abstime _time_wp_reached{0};