west: runners: nrf: Check for missing UICR

On nRF54H and nRF92, booting certain cores requires programming a UICR,
which is normally generated using nrf-regtool. This should be considered
an optional dependency, because we do not wish to force non-Nordic users
to install it just to work with Zephyr, or just for build-only tests.

When nrf-regtool is not installed, a CMake warning will be displayed,
but people ignore warnings all the time. As the last line of defense,
check for missing UICR in the nrfutil flash runner, to prevent our users
from unintentionally programming unbootable firmware. Show a fatal error
specifically if CONFIG_NRF_REGTOOL_GENERATE_UICR=y, yet no UICR exists.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>