bug#21843: Generated grub.cfg does not support encrypted roots

[Ludovic Courtès]

As reported by 宋文武 at
<https://lists.gnu.org/archive/html/guix-devel/2015-11/msg00096.html>:

  Follow the manual to setup encryted root, using the desktop.scm
  template, but
  at the final step, it failed with:

    Path '/mnt/boot/grub' is not readable by GRUB on boot.
    Installation is impossible. Aborting.

    (can be reproduced by `grub-install /dev/sdb --boot-directory
  /mnt/boot')

  After search, it seems that an un-encrypted boot partition is needed:

    
https://wiki.archlinux.org/index.php/Dm-crypt/Encrypting_an_entire_system#Preparing_the_boot_partition

  So, I run fdisk to add a boot partiotion, finally install finished!

  But it can't boot, I have to mount the encrypted root in Grub's cmdline:

    insmod cryptodisk
    insmod luks
    cryptomount hd0,msdos2

Ludo’.