cmake: use CMake variable KERNEL_VERSION_CUSTOMIZATION for default value

Fixes: #80193

With #62395 the Zephyr kernel and app version customization values were
moved to target properties to allow Zephyr modules to adjust the values.

This had the consequence described by #80193 that version customization
using CMake arguments, `-D`, or CMakeList.txt toplevel file can no
longer be used for customizing the version.

To support both CMake variable as well as Zephyr module version
customization use-cases then this commit uses `zephyr_get()` to fetch
any CMake variable adjustments and uses the value for default property
setting. This allows users to set customized version while still allow
Zephyr modules to overrule this as intended with #62395.

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