fs/iso9660: add hybrid image support

This commit adds a new option, which allows, when isolinux is used as
the bootloader, to generate an "hybrid" ISO image. Such images can
either be booted from CD-ROM or from USB keys. It simply uses the
isohybrid tool provided by syslinux.

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