New cmdline option -S + env variable SCRIPTS_DIRECTORY to set script directory

This allows us to run grml-live from the Git repository without
having /usr/share/grml-live/scripts available.

Thanks: Krisztian VASAS <iron@ironiq.hu> for the initial patch
Possibility to give LIVE_CONF as env. variable
Don't use addons in GRUB if NO_ADDONS is set
Fixes to be able to build Grml based on Debian Wheezy

In Debian Wheezy initramfs-tools is 0.109, but the Grml kernel needs
at least 0.110. The backport contains the requested version.