Fix File Not Found error in GRUB2

When loading grml-plus via BIOS, shortly before the menu appears there is
an "error: file not found." error shown.

This is caused by the missing gzio.mod file.

Fix it by copying it into /grml-plus/grub.
Add Netboot support

This addon adds two entries to the grml-plus menu:
* boot via netboot.me (using their custom GPXE build)
* boot from LAN (via IPXE included in GRML)
[Incomplete] Add Ultimate Boot CD

This addon is still incomplete; most of the menu items are not yet
working. But since I underestimated the effort needed to parse their menu
file and create a GRUB2 config for it, I'll commit the preliminary version
now and update it when I found time to complete it.