I wrote a modified version of wiring.c
By adding this wiring.c file to your sketch folder or by putting it in into the Arduboy(2)\src folder, a special feature will be added to your sketch:
By pressing the UP + DOWN buttons (and no other buttons) for approxemately 1.5 seconds will exit the sketch and enter the FX Loader menu (bootloader mode)
interrupt service routine for timer0 is rewritten in assembly and optimized.
consumes 8 bytes of extra PROGMEM
frees 3 bytes of ram (1 byte extra ram used. But ISR uses 4 less stack pushes)