conf-files: add flag so that we don't always prefix returned paths with the root...

conf-files: add flag so that we don't always prefix returned paths with the root dir path used

This is useful in tools such as system-repart where we show the
definition file paths a lot in our output, but if prefixed with the root
path we'd show a temporary mount dir when operating on a image file.
Hence, let's drop the prefix here, and show only the path within the
image.