fs/iso9660: get grub splash from $(TARGET_DIR)

Instead of using directly the splash image from the Buildroot source
directory boot/grub, this commit changes the iso9660 logic to use the
splash image installed in $(TARGET_DIR)/boot/grub.

This effectively allows a user to use a custom splash image by
installing it to $(TARGET_DIR) through a rootfs overlay or using a
post-build script.

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