Plane: removed "circle waypoint" completion test

this is redundent with the location_passed_point() test
Plane: explain verify_*() commands
Plane: made variables static and remove initial rc overrides

minor code size savings
Plane: move elevon variables into a structure
Plane: use RADIUS_OF_EARTH define
AP_L1_Control : Fixed potential divide by zero in Nu1 angle calculation
Plane: fixed LOITER_TURNS command

and cleanup more loiter variables
Plane: fixed LOITER_TIME to start when loiter starts

thanks to Paul Riseborough for finding this bug!