Recent changes to makefs and mkimg have led to situations where the
disconnect between this script and the versions installed on the host cause
failures. Provide a way to work around this that doesn't require the
installation of new versions to the host system if that's not desired.
With this change mkisoimages.sh will honour the $ETDUMP, $MAKEFS and $MKIMG
environment variables but fall back to the previous behaviour of finding them
within $PATH.
Reviewed by: gjb
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D15181
498102ca2b