Use debootstrap with --no-merged-usr by default

By default debootstrap creates systems with /usr being merged, leading
to issues like the xfsdump issue we saw in
https://github.com/grml/grml-live/issues/85, also dpkg -S ... and
further commands break with /usr being merged.

It's possible to generate custom basefiles with debootstrap, but by
default we should invoke debootstrap with the --no-merged-usr option,
otherwise we might have different ISO layouts, depending on whether we
use appropriate basefiles (as on our offical build host) vs. locally.

Closes: grml/grml-live#86