mk-s390-cdboot has stopped working because the kernel outgrew the
hard-coded offset it used when creating cdboot.img. IBM now has a script
in s390utils that can do the same thing so use the upstream script
instead.
This drops mk-s390-cdboot script, switches the s390 templates to use
mk-s390image from s390utils.
It adds @ROOT@ to cdboot.prm, and sets inst.stage2 so that the installer
image will be found when booting the iso.
Related: rhbz#1892404
