fs/iso9660: add Grub splashscreen support

When Grub is built with splashscreen support, copy the splashscreen
image to the ISO9660 filesystem. Otherwise, disable the splashscreen
in the grub menu.lst file.

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