So it turns out that BusyBox doesn't auto-load modules for filesystems
that have type "auto", e.g. it doesn't figure out that it should load
the "iso9660" module should be loaded when mounting the CD-ROM. We
don't want to give the root FS on the CD type "iso9660", because that
breaks booting from a USB stick created by unetbootin. So make sure
the "iso9660" module is loaded.