grub-editenv: Warn a user against editing environment block

grub-editenv: Warn a user against editing environment block

The environment block is a preallocated 1024-byte file which serves as
persistent storage for environment variables. It has its own format
which is sensitive to corruption if an editor does not know how to
process it. Besides that the editor may inadvertently change grubenv
file size and/or make it sparse which can lead to unexpected results.

This patch adds a message to the grubenv file to warn a user against
editing it by tools other than grub-editenv.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>