Ensure GRUB finds the running ISO

Instead of relying on the presence of the /conf/bootid.txt file, which
might be present on several Grml ISOs, let's generate a unique filename
which GRUB then can search for.

JFTR: The redirection within the layers (scripts/GRMLBASE/45-grub-images
vs main grml-live script) is necessary, because the GRUB image is
generated from within the rootfs (grml_chroot), but the rootfs is
compressed within a squashfs file, while we can only access files
available directly inside the ISO (corresponding to the files inside
grml_cd, and not the files inside grml_chroot which
scripts/GRMLBASE/45-grub-images has access to).