Merge pull request #418 from matthijskooijman/arduino

Some additional improvements for Arduino
do not define __AVR_ATmega2560__ manually

Passing -mmcu to the compiler already does this
Hack to compile for the Pinoccio Scout

This expects the hardware/arduino/avr directory to be copied or linked
as targetlibs/arduino_avr and hardware/pinoccio/avr as
targetlibs/pinoccio_avr.

Note that this doesn't actually see
Disable DEBUG, that greatly increases binary size