iso9660 fs: use cpio initrd instead of ext2

The ext2 initrd is pretty broken, because it requires an additional
root=/dev/ram0 command line parameter, and a /init to mount
devtmps that isn't there in out ext2 rootfs.  So just use a cpio
instead.

Note that there is no check if the kernel supports initramfs or the
selected compression method.

Also removed a bit of dead code in iso9660.mk.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>