Initial vfat support.

[Marius Bakke]

Hello Guix,

These patches allow using a FAT32 /boot partition.

There are a couple of problems still:

* grub-efi tests are failing. I don't understand why, any tips here
  appreciated. This package can wait, however.

* The changes to base-initrd are only picked up if (needed-for-boot #t).
  I think this is a bug, since the initrd wants to check all defined
  file systems, and /boot is not actually needed for Linux boot.