LB_DEBIAN_INSTALLER_GUI defines whether or not to provide the graphical
installer. the installer_debian-installer script pays attention to it and
does not download it if not wanted. the actual bootloaders however
(both grub2/loopback and syslinux) ignore it, which leaves broken and
unwanted menu entries. this fixes that.
Gbp-Dch: Short
Closes: #952890
Lyndon Brown