thanks to unionfs. For instance, nix-env and nixos-rebuild work.
The tricky part was to build a Nix database (in the tmpfs/unionfs)
which is now necessary to prevent store paths on the CD from being
deleted right away because they otherwise wouldn't be valid.
* nixos-install: use the /etc/nixos/configuration.nix from the target
file system (don't copy it anymore). Since the user is supposed to
mount the target file system on /mnt anyway, we may as well require
that configuration.nix is placed in /mnt/etc/nixos. This also makes
upgrading / reinstalling much easier, since it will automatically
use the right configuration.nix.
svn path=/nixos/trunk/; revision=10399
