arduino_mkrzero: add blinky_rtic example

[arkap]

Summary

Add a blinky_rtic example for arduino_mkrzero.

I've adapted the example from the feather_m0 board for the arduino_mkrzero board. It's currently throwing a lot of warnings because of the use of gpio::v1.

[TDHolmes]

Another one waiting on #570

[bradleyharden]

@arkap, just pinging you about this, since #570 was merged.

[sajattack]

Superceded by #697