default to entry 0 if no saved_entry is present in grub2 env

Fixes: bnc#773143

Signed-off-by: Andrey Borzenkov <arvidjaar@gmail.com>
Merge pull request #14 from arvidjaar/fu/grub2-efi-editenv

default to entry 0 if no saved_entry is present in grub2 env