soc: nordic: uicr: Reorganize how gen_uicr.py is invoked

Reorganize how gen_uicr.py is invoked.

Instead of invoking it from one of the Zephyr images we invoke it from
a new special Zephyr image called uicr.

This uicr Zephyr image is flashed in the same way as normal Zephyr
images so special handling in the runner is no longer necessary.

Also, we simplify gen_uicr.py by moving parsing of Kconfig/DT from
gen_uicr.py to CMake.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>