- bit of clean up
- add GPS failsafe mode auto_altitude, that will keep current altitude with a fixed-bank angle
for some time, then switches to auto_descend that will descend with constant sink rate
of 0.5m/s
- params controlling GPS failsafe are not FW params: NAV_GPSF_R --> FW_GPSF_R and
NAV_GPSF_LT --> FW_GPSF_LT
Signed-off-by: Silvan Fuhrer <silvan@auterion.com>