Move AVR core to its own repo

Solves #5976; the new core's home is https://github.com/arduino/ArduinoCore-avr

Most of the history has been moved and is accessible, however the first 2 years are too much noisy (and SVN based) to be able to extract anything useful.

Please refer to Arduino main repo if you are doing any informatic archaeology :)

Author: facchinm