Plane: initial code for RTL breakout to land

Modified ArduPlane.pde: Inside of update_flight_mode added code to RTL case that checks (1-3)
and if true breaks RTL and goes to land. Added a static RallyLocation best_rally_loc thats used for flag checking.
   1st) If the rally flag for autoland is set
   2nd) If our heading is directed towards a landing wp
   3rd) Were within a +/-20meter tolerance of our break_alt

Modified rally.pde: Inside rally_location_to_location if rally flag for autoland is set, changes
the return location to the break_alt.

Modified commands_log.pde: When do_RTL() is called best_rally_loc struct is set.