btrfs detection for grub and associated btrfs cleanup

Merged dalto requested to merge btrfs_detection_fix into master

Replaced the checking tmp/.btrfs with a call to findmnt. This is the only place that file was being used so all the references to it were removed. Also replaced some hardcoded /mnt with $MOUNTPOINT

If we only want the changes to grub it would be safe to not merge the changes to util-disk.sh

Merge request reports