Copter: relax altitude check in verify_takeoff

Previously we checked if the alt > target_alt but because there is
little to no overshoot with the new accel based alt controller this
check was failing for some users.
Copter: added experimental LOITER_REPOSITIONING #define to config.h

APM_Config.h should be used to overriding the standard parameters but
they still require a definition in config.h or they will be undefined
when built by the mission planner.
Copter: constrain loiter so that it never commands more than 45 degrees of lean in lat or lon directions