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 <Torsten.Rasmussen@nordicsemi.no>