This patch changes grub_env_export to create variables with empty value, 
if necessary.
This makes it possible to export variables before actually assigning 
them any value and is the way bash behaves.

Thanks
Andreas

ChangeLog:
2009-10-15  Andreas Born  <futur.andy@googlemail.com>

        * kern/env.c (grub_env_export): Create inexistent variables 
before exporting.