lib.sh: base image: Prevent using incomplete img

lib.sh: base image: Prevent using incomplete img

In case the previous deploy attempt already copied the base image
as the VCP image in order to perform offline operations and failed,
leaving an incomplete image in place, current code might try to use
it instead of building it from scratch.

Use the hash-agnostic link names as checkpoints for successful image
handling.

Change-Id: I1e99e515e18ba1dec534c520811c127b2b528afe
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
(cherry picked from commit 5878c5c0c2ca8c78e9a5124452eefe8290ab63cd)