cmake: Zephyr kernel version.h and app_version.h creation

Change the creation of version.h and app_version.h to apply the
KERNEL_VERSION_CUSTOMIZATION APP_VERSION_CUSTOMIZATION values as target
properties instead of CMake variables.

This allows more freedom for users / Zephyr modules to adjust the value
of KERNEL_VERSION_CUSTOMIZATION and APP_VERSION_CUSTOMIZATION values and
thereby make use of the functionality introduced with #61635.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>