fs/iso9660: change the location of the splash image

In order to simplify the introduction of the support for using ISO9660
as the real root filesystem, this commit changes the location of the
grub splash image. This makes it match where the splash image is
located in $(TARGET_DIR), so that regardless of whether the
initrd/initramfs solution or the real iso9660 filesystem solution are
used, the splash image is installed at the same location.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>