Add more filesystems to modular GRUB2

This fixes the bug described in http://bts.grml.org/grml/issue1306
that a grml2usb prepared medium cannot boot from (U)EFI.

Added a few more filesystems just in case somebody wants to use
grml2usb with a non-mainstream filesystem.

I am unable to reproduce the second bug in that issue, when dd-ing
the ISO to a USB device - this scenario works fine with the (U)EFI
implementation used by VirtualBox.