mega2560: Implement TWI for debugging over battery SMBus connection

[crawfxrd]

Implement TWI (two-wire interface) for the Mega 2560 for debugging over the battery SMBus connection.

This would remove the need for the Adafruit Trinket M0.

[jackpot51]

There's a voltage mismatch that could kill everything, our motherboards have 3.3V busses but a mega2560 has a 5v bus