fs/iso9660: do not use an initrd in iso image when using initramfs

When using initramfs built into the kernel, it is useless to put an
initrd in the iso image.  This patch makes the image to only contain
the kernel image, and also removes the initrd line from menu.lst

Signed-off-by: Thierry Bultel <thierry.bultel@wanadoo.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>