This is just the functionality of what's already documented in the
description of initlabel.

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

Every device specification is in the form "<type>.<name>" and so far the
type for referencing a specific device has been a plain types.str.

Now we're not only checking whether the device specification is a string
but also whether its syntax is correct and the type actually exists and
is valid for a particular option.

We now have a deviceTypes attribute set which is our main definition for
all available device specifications and it also categorizes them with
attributes like "resizable" or "orderable" which add the corresponding
options to the option set of the device specification submodule.

What's still missing are assertions on whether the actual name
references a device which actually has been defined.

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

I initially had these options in the storage module before actually
adding them to <nixpkgs>. Now it's time to put them back into the
storage module so that we have everything that's related to the module
in one place, so that we can do even more comprehensive type checking.

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

The sizes relative to the corresponding smaller units.

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

Whenever a device specification is cross-referenced we need to check
whether a definition for the exists. So for example if we have:

storage.mdraid.raid.devices = [ "partition.raid1" "partition.raid2" ];

We need to make sure here that storage.partition.raid1 and
storage.partition.raid2 are actually defined.

Of course we could check this within nixpart as well, but we want to
avoid such errors at run time.

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

This is an exception to the container types (isContainer in
deviceTypes) in that we *only* allow fsType to be "btrfs" for btrfs
subvolumes.

If this is set to something else than "btrfs", throw an assertion error
printing the conflicting options.

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