cmake: update zephyr_file to use cmake_path()

Since the introduction of zephyr_file() then the minimum required CMake
version has become 3.20.
Update zephyr_file() to use cmake_path insteasd of set().
This allows us to normalize the path and thereby also remove duplicates
in the list as paths are normalized.

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