Encrypted root partition

[Ludovic Courtès]

Hello Guix!

In case you missed it, GuixSD now supports LUKS-encrypted root
partitions, finally!

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=21843#48

And there’s a system test, which can also serve as an example for people
who want to test it:

  http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/tests/install.scm#n392

The test is pretty fun because it resorts to OCR on QEMU screenshots to
determine when it can enter the passphrase (when booting the installed
system).

  http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/tests/install.scm#n456
  http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/build/marionette.scm#n177

It should turn green on Hydra when it gets to run it:

  https://hydra.gnu.org/job/gnu/master/test.encrypted-root-os.x86_64-linux

In the meantime, you can always run:

  make check-system TESTS=encrypted-root-os

Now we need documentation.  Petter had submitted a patch back in
February(!) so I guess I’ll start from there and see if any adjustments
need to be made.

Feedback welcome!

Ludo’.