Use mkisofs to create ISO images

mkisofs is the original ISO creation tool from cdrtools available on
many UNIX systems. genisoimage on the other hand is part of a mostly
unmaintained fork of cdrtools very specific to Debian Linux and its
derivates (e.g., Ubuntu). Fortunately, genisoimage (as we used it) is
completely invocation-compatible to mkisofs.

Fixes #627.