bug#25568: LUKS on top of RAID not supported

[Ludovic Courtès]

Having a root partition on LUKS on RAID is not supported as of 0.12.0,
as reported by Divan Santana <di...@santanas.co.za>:

  https://lists.gnu.org/archive/html/help-guix/2017-01/msg00047.html

Ludo’.