grub2: prepare and install El Torito image

In order to support ISO9660 bootable images that rely on Grub 2, this
commit modifies thr Grub 2 makefile to generate and install an El
Torito image. Such an image is simply produced by concatenating the
cdboot.img provided by Grub 2, and the Grub 2 image generated by
Buildroot using grub-mkimage.

Since this action is so simple and cost-free, we don't bother adding a
Grub 2 sub-option for that, and simply generate the El Torito image
unconditionally.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>