Do the same as DO_REPOSITION wit only the altitude field populated
and MAV_DO_REPOSITION_FLAGS set, which means:
- switch to Loiter mode if not already in it
- set the current altitude to what is specified in the altitdue field,
keep current altitude setpoint otherwise
- keep current position setpoint
- fall back to current estimated position in case a position setpoint
is not finite
Signed-off-by: Silvan Fuhrer <silvan@auterion.com>