scripts: runners: nrf: Add support for the new nRF Util tool

Add support for Nordic's new nRF Util tool, aka nrfutil. Via its
"device" command, nrfutil now supports most of the functionality offered
by nrfjprog.

The tool itself can be found here:

https://www.nordicsemi.com/Products/Development-tools/nrf-util



Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>