Allow EXTRA_DEB_REPOS to multistrap general repos

In before it was only working if all the repositories in EXTRA_DEB_REPOS
were from the OSCI-VM repos.

Closes-Bug: 1381672
Change-Id: I25806855cad76e8f7b106a778bf1c80c85555a16
Merge "Allow EXTRA_DEB_REPOS to multistrap general repos" into stable/5.1