Get rid of code which breaks Multi-ISO boot [Testing: issue1273]

Removed lots of code which is not needed any longer because grml2usb
can do a lot of the work.
Did some redesign to support multiple ISOs, see also bug report #1273.
Because of this changes it's no longer possible to overwrite the used
kernel in /etc/grml/terminalserver/config. This should not be needed
nowadays, anyway.

Fixed syntax error and wrong variable assignement

In default_config the variable MULTIBOOT was assigned wrong
Remove old grub support

Removed code to generate menu.lst in tftp root dir. grml2usb generates
grub config now and also takes care of live-media-path.
grub-pxelinux_config: use shell features for directory browsing