altos: Merge GPS logging into a single function

Create a new global, ao_gps_new, which indicates new GPS position and
satellite data.

Use ao_gps_new as the new sleep/wakeup address.

Merge the separate gps position/satellite logging tasks into a single
function which waits for new data and writes out the changed values.

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