ade: Fallback to /etc/os-release

Tool ade needs to find out release and distribution information for SDK and
sysroots. This was previously done reading /etc/image_version but this
custom file was dropped in favor of /etc/os-release.

In order to support both the old and the new approach fallback to
/etc/os-release if /etc/image_version is not found.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>