altos: Add tiny logging for TeleMini/TeleNano

This splits the logging code into management of the log space within
storage and separate code to actually write suitable log entries.  A
new log writing module, ao_log_tiny, is added which writes only
altimeter data at a fairly low data rate for devices using on-chip
storage.

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