grub2: build El Torito image only for i386 PC platform

Building Grub2 El Torito for i386 EFI errors complaining it cannot
find cdboot.img

        cdboot.img: No such file or directory

This commit builds El Torito for i386 PC only.

[Thomas: fix installation ordering.]

Signed-off-by: Ray Kinsella <ray.kinsella@intel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>