fs/iso9660: doesn't support (grub2) EFI

fs/iso9660: doesn't support (grub2) EFI

The iso9660 generation for grub2 assumes that grub-eltorito.img is
available. However, this image is only available for the i386-pc target
(i.e. legacy BIOS). An EFI-bootable iso9660 requires a different layout.

Since we currently can't generate the EFI-bootable iso9660 layout,
require the i386-pc target to be selected in grub.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reported-by: arnaud.miche@orange.com
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>