add code for the atmega8u2 HAL

[drmorr0]

I haven't done much testing on this yet; we need to merge the changes to avr-device in first.

[Rahix]

Ah, sorry, I didn't make it clear enough I guess :( We're currently in the process of reworking almost the entirety of avr-hal. See issue #130 for details. This work is happening in the next branch in this repository and I would suggest that you add the new chip ontop that branch because anything on master won't be here to stay for that much longer. I.e. you'll have to open a PR against the next branch and do changes similar to other PRs which were part of the refactoring. Most are linked in issue #130, but as a good examples, take a look at #145 which adds support for ATmega168.

Sorry for not explaining that better from the get go :/

[drmorr0]

@Rahix no worries! This wasn't too hard to put together, I just wanted some validation that the avr-device change was correct. I'll work on getting this into the next branch now :)