set in src/etc after a "make release" in both "src" and "xsrc", to create a
ISO-image of the release in $RELEASEDIR/installation/cdrom.
Hook for architecture dependent pre/post-processing in etc/etc.*/Makefile.inc
are available as 'iso-image_md_post' and 'iso-image_md_pre', see
etc/etc.i386/Makefile.inc as an example. Might be useful for setting up
bootable CDs on alpha, sparc, ...
Reviewed by Todd Whitesel and Thomas Klausner.