cmake: strip paths in build output based on Kconfig option

Add CONFIG_BUILD_OUTPUT_STRIP_PATHS to allow keeping absolute paths
in the build output.

This can be helpful for IDEs to detect links in a console output and
jump to the code location (e.g. when running tests in native_posix
locally).

Debuggers usually have a path mapping feature to ensure the files are
still found.

Signed-off-by: Martin Jäger <martin@libre.solar>