cmake: introduce zephyr_file() extension command
cmake: introduce zephyr_file() extension command

Fixes: #23825

This commit introduces a `zephyr_file()` extension command that allows
for examine a path list for relative paths, and convert relative paths
provided with `-D<path>` into absolute paths, starting from
`APPLICATION_SOURCE_DIR`.

Signed-off-by: Torsten Rasmussen <a7c5b1d5a1f41fce6edfa3f7c2392ae0c59deafe@nordicsemi.no>

Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr