grub, bootloader: Make the grub-* and /boot/grub names configurable.

Some distributions (e.g. Fedora) use grub2-* (resp. /boot/grub2/) names.

Fixes #123.
Merge pull request #150 from kkofler/fix-issue-123

grub, bootloader: Make the grub-* and /boot/grub names configurable.