nixos-generate-config: Add --show-hardware-config.

So, we get the old behaviour of nixos-hardware-scane if we run the
following command:

nixos-generate-config --no-filesystems --show-hardware-config

This allows to use scripts in order to fetch NixOS specific hardware
information, without the need to duplicate code elsewhere.

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