generate /etc/image_version

Few tools such apertis-eclipse-plugins or ade need information
provided by /etc/image_version file. Moving into `debos` caused
this file to be removed, which is a long term goal, however we
need to adapt the tools first, but we are not in time for this
release (17.12).

Signed-off-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7378