Add option to print NixOS config to stdout.

Previously we were _always_ writing the NixOS config to stdout. But that
doesn't always make sense, especially while just mounting filesystems.

So, we don't output anything now, unless explicitly requested by -p.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Include GRUB devices in NixOS config, closes #1.

Add all disklabels and BIOS boot devices to the NixOS configuration
printed with -p.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Workaround to give pretty grub devices output.

In NixOS we can nest attributes by ".", so it doesn't make sense to spit
out a whole bunch of nested curly brackets just for the grub option. So
unless we don't emit other options, just allowing . and hardcoding the
nested attrset will do it for now.

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