AP_Mission: Use current_loc rather than AHRS for setting previous waypoint when commands are changed. Altitude to hold RTL is now a AP_Mission parameter to allow proper setting of home altitudes for end of AUTO mission and RTL.
AP_Mission: Bug fixes related to non_nav commands at the very end of missions.
Plane: Updates because AP_Mission now handles RTL altitude parameter and current_loc rather than AHRS for setting previous waypoint
Plane: non_nav commands won't be executed unless in AUTO mode.
AP_Mission: Added additional method comments and cleaned up formatting.
AP_Mission: Consolidate biasing of home altitude and nav id
AP_Mission: Change get_mission_status() to get_status() for clarity.
Plane: Changed mission.get_mission_status() to mission.get_status() to reflect change in AP_Mission.
Plane: make sure new waypoints are only processed when the mission status is good (true)
Plane: After takeoff, ensure previous waypoint is set after waypoints have been incremented