swap timer0 for symbol counter for millis and micros calculation

[jacobrosenthal]

This allows us to keep proper millis and micros even while asleep! Thus also allows systimers to keep proper time while asleep

[soldair]

awesome

[soldair]

@matthijskooijman do you think he needs all of these avr/* files?

[matthijskooijman]

Hm, I'm not too thrilled about this change. I like the end result, but to achieve it, a full copy of the Arduino core is made, to allow changing the millis() function. If we really want to achieve this, I'd prefer looking more closely at the Arduino core and see if we can contribute a change to Arduino upstream to allow making such a change from a variant (though that will take a lot more time to get accepted in Arduino, of course...).

Also, I'm not so sure if making millis() count during sleep is the best approach - it doesn't work like that on other boards, so this could cause portability problems for sketches and libraries (though I think there is not much code out there that takes sleeping into account at all).