Make sure to not leak any mdadm configurations from the build system

We haven't run into this issue yet, though it was reported
on the debian-live mailing list:

  https://lists.debian.org/debian-live/2014/03/msg00063.html

and we don't want to have anyone using grml-live run into it.

Thanks: Evgeni Golov for the pointer
Reviewed-by: Evgeni Golov