Fix validation when no image is present

The slot validation routine would result in a non-installed image to be
equivalent to a failed validation, which would set the swap type to
*_FAIL and require special handling in the dependency checks. This
routine was updated, so that it now returns a value that can be used to
check that the error was due to an image not found.

Signed-off-by: Fabio Utzig <utzig@apache.org>