if no snap found, menu generate correctly, and show a warning
- overhaul
- add two new custom variable to /etc/default/grub :
* GRUB_BTRFS_SHOW_TOTAL_SNAPSHOTS_FOUND="true"
(Show Total of snapshots found during run "grub-mkconfig")

* GRUB_BTRFS_SHOW_SNAPSHOTS_FOUND="true"
(Show snapshots found during run "grub-mkconfig")