Unless '-ignorestderr' is set on 'exec' in expect, any output on stderr
is interpreted as execution failure. In this case, 'create_iso' logs
some diagnostics but does not fail.
We repeatedly promote our ISOs to be bootable from USB storage.
Therefore, we have a hard dependency on 'isohybrid' and should fail in
the case that the tool is missing.
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.
