nixos: Create firmware output path without links.

Especially when "${kernel}/lib/firmware" is referenced, the store path
for firmware could get quite large (>= 150 MB on x86_64-linux), and for
example in VM tests this closure will end up being included in the
initrd.

Instead of building up a link farm, we now just dereference/copy over
all files into the firmware closure.

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