altos: Set TeleGPS default log size to half of avaiable memory

altos: Set TeleGPS default log size to half of avaiable memory

LPC11U14 doesn't have on-chip config space, so we're consuming a block
of external flash. As a result, only 1984kB are available for
logging. Set the default log size to half of that to store two flights.

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