fs/iso9660: select ISO9660 support in Grub

The fs/iso9660 logic assumes that the Grub bootloader is
used. Therefore, it should make sure that Grub is configured with the
support for the ISO9660 filesystem.

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