scripts: west_commands: runners: nrf_common: optional UICR cleanup

The flasher was unconditionally cleaning the UICR area, even when the
application didn't have a new configuration generated. This can happen,
when CONFIG_NRF_REGTOOL_GENERATE_UICR=n. In such case, keep the old UICR
configuration on the device.

A real scenario where we should set CONFIG_NRF_REGTOOL_GENERATE_UICR=n
is when building multiple firmware images that are meant to run one
domain. The primary application build generates the UICR configuration
and secondary images don't. Before this change, the flashing process of
the primary application would write new UICR configuration, but the
flashing process of secondary images would erase it.

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>