altos: Make micropeak 'serial' interface work

I prototyped the mpserial interface on a breadboard and tuned the
circuit to register the LED correctly. Then adjusted the serial code
to send bits at the right speed and format.

The logging contents are now in hexdecimal with a CCITT CRC-16
computed to verify correct reception.

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