altosui: Use GPS alt + baro height for KML altitude

altosui: Use GPS alt + baro height for KML altitude

GPS altitude is generally more absolutely correct than baro altitude,
so use that as the nominal pad altitude when generating a KML
file. This results in a KML file that has the flight trace start and
end closer to the ground, which is always nice.

Signed-off-by: Keith Packard <keithp@keithp.com>