altos: Add TeleGPS logging format

This is mostly like the mega format, but places the flight state in a
spare byte of the GPS data and writes the gps starting location to the
flight packet.

Log data is written by the main tracker thread; there's no reason for
a separate thread given the GPS update rate and the lack of flight
controls. This means ao_log_gps has an API to be called from there,
rather than a thread to run.

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