Add a simple I2C protocol alternative to MK-BLconfig stuff.

This one works like most I2C slaves, where there's a register map,
address byte is transmitted first and the addresses get auto-incremented
while you keep reading or writing.

There's a write-only register for throttle, and four read-only register
for RPM data, Vbat, Temperature and the ID byte.
Defer the comparator multiplexer switch until needed.

This allows us to use the ADC whenever we're not using the Analog
Comparator.  Without it we could only use the ADC on the ESC boards that
connect all three phases through the multiplexer (not AIN1) and so only
ADMUX needed to be modified when the phases were switched.  Since ADMUX
is buffered it didn't disturb the ongoing conversion.
Define ADC reference voltage per board.
Define the I2C slave address in the more usual form.

I.e. not shifted left.
Set MOTOR_ID on command line instead of in the code.

Also make it 0-based for simplicity (ok, that's quite arbitrary).
Makefile target to build .hex's with MOTOR_IDs 0-7.

The build_8 target produces files name .hex.0, .hex.1 and so on, with
the corresponding MOTOR_IDs hardcoded in them.