altos: Add Pico Beacon code as ao_aprs.c

Pico Beacon hooks a GPS to an AD9954 DDS radio chip with a PIC. It
directly synthesizes the necessary AX.25 packets to do APRS
reporting. We're going to appropriate the code for use in Mega Metrum
to (optionally) broadcast APRS packets.

	http://ad7zj.net/kd7lmo/aprsbeacon_code.html

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