Allow to build custom chroot
This allows to build custom chroot with extra repositores. This setup is being used by our CI repos but was never committed, this fixes it so we can also setup the plamo repos.
@bob.ham this does not yet include the "apt-get update" stop but you should be able to start building the first packages now when setting dists: dists: ['buster+plamo'] in deb-build-jobs.