Restores the pre-PR latch: takeoff_expected is set while armed and
land_complete and only cleared by the release check, so an altitude
release is not undone by a later dip below GNDEFF_ALT and modes that
disable it (THROW) never see it in the air.
Also clamps GNDEFF_TMO before the unsigned conversion, only trusts the
takeoff XY anchor for the drift gate when it was captured with a valid
horizontal position, and renames the drift define to say it is a NE
distance.
Vehicle-agnostic detector that tells the EKF when to expect baro
disturbance from rotor downwash near the ground, driving AP_AHRS
set_takeoff_expected and set_touchdown_expected. Adds GNDEFF_ALT
(altitude threshold and master enable) and GNDEFF_TMO (takeoff hold
time).
Randy Mackay
Andy Piper