Fix installing grub when /boot is inside the root filesystem
This fixes simplifies the code by using df in order to find the filesystem name (mountpoint) of a path and using this to determine whether /boot is in the root filesystem.

This change patches both grub and grub2 (since it was copied from grub).

This fixes #210.
Merge pull request #237 from dagwieers/boot-grub-fix

Fix installing grub when /boot is inside the root filesystem