Populate Hafnium image name based on dynamic values

Avoid hardcoding the hafnium image name and instead use folder values
to create the image name. Also suffix the 'ubuntu' string into the
image following the new image name nomemclature from parent commit, so now
the image name would be trustedfirmware/ci-amd64-hafnium-ubuntu:bionic
instead of trustedfirmware/ci-amd64-hafnium:bionic.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I206b35eb9f8c897e6b63eef7d733fee8e3db8083
1 file changed