grub: Add mdadm to search path.

Grub uses mdadm to find out the device it is on, especially when mdadm itself
resides in a separate boot partition. When bootstrapping from a NixOS
installation CD, it's not a big issue because usually the paths from the Nix
store of the installation CD are matching with the ones in the chrooted
environment.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>