This allows specifying a path outside of the data directory using an
absolute path (e.g. /path/to/devices.yaml) or a path relative to the data
directory (e.g. devices.yaml, foo/devices.yaml, ../../devices.yaml, ...).
This change is backward compatible because file names and nested paths are
just a special case of a relative path.
Co-authored-by: Koen Kanters <koenkanters94@gmail.com>