boards: nrf: Default to nRF Util for flashing

Nordic has a new tool, nRF Util (`nrfutil`) which is replacing the
previous set of command-line tools (particularly `nrfjprog`) that Zephyr
had been using by default in many of the boards to flash the firmware
onto the device.

Since nRF Util is already mature and is going to be replacing the existing
tools, including `nrfjprog`, it is now a good time to change the default
flash tool to reflect this fact.

Note that `nrfjprog` support is not being removed at all, but this may
be a breaking change for users that do not have nRF Util installed. For
this reason we also add a note in the migration guide.

More info:
https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/nrf-util-unified-command-line-utility

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

Documentation:
https://docs.nordicsemi.com/bundle/nrfutil/page/README.html



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