AX25 Modem on AVR Devices

This started off as a fork of https://github.com/slepp/AX25 but because I'm going in a different direction with the code, I'm not sure if that was a good idea.

I plan to merge the code from my kiss-avraprs repository to remove the dependency on Bertos and allow use of more interesting chips, like the ATMega32U4.