append `./` when checking image_version file

sysroot tarball packs `./etc/image_version`, ade fails
to identify it as `etc/image_version`, therefore, we
need to append `./` to the file path.

Signed-off-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.com>
Differential Revision: https://phabricator.apertis.org/D7384