How to enable PDI upload

[zdjurisic]

What does it take to add a PDI upload option? Using mattaita4u, in boards.txt I (naively, probably) changed upload.tool to arduino:avrdude and upload.protocol to wiring (which works with XmegaForArduino cores and my mkII programmer). That did not work and instead threw java.lang.NullPointerException. There is probably more to it... If that matters, I am using Arduino IDE 1.8.2.