Tools such as efibootmgr rely on the deprecated /sys/firmware/efi/vars API as
well as on the new /sys/firmware/efi/efivars API. The latter needs to be
mounted.
Reported by Keyhenge here:
https://lists.gnu.org/archive/html/bug-guix/2020-04/msg00274.html
Here is the efivarfs documentation:
https://www.kernel.org/doc/Documentation/filesystems/efivarfs.txt.
* gnu/system/file-systems.scm (%efivars-file-system): New exported variable,
(%base-file-systems): add it.
* gnu/system/install.scm (%efivars-file-system): Add it.
Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>