[avr] Add option to use TIMER1 with a 4usec resolution

Thought I'd give this a try to see if it provides better timing but
it doesn't appear to be any better or worse than micros() and is harder
to read so disabled it.
[avr] Add support for RF69 module. RX moved to pin 8
[avr] Remove TIMER1 timing because micros() works just as well