Port build-image.sh from old config repo

Port the build-image.sh script, build-dib-in-docker.sh, mount-image.sh
and umount-image.sh into the new repo, adjusting the paths for the new
layout

Change-Id: I087a2396581bf4415c9676f70b4c739a62c84b23
Merge "Port build-image.sh from old config repo"