Add README.md for ev-aducrf101mkxz platform
Add IAR compiler support for ADuCRF101
Add Windows build instructions for ev-aducrf101mkxz platform
Fix radio hangups when trying to transmit with radio off.

The border-router tries to transmit and do other stuff after turning
the radio off, and the radio driver didn't handle that very well.
With this fix, it's no longer necessary to reset the border router
after starting tunslip6.
Add modern ARM compiler and ev-aducrf101mkxz to travis tests