- move guts of distrib/Makefile.inc to distrib/common/Makefile.distrib
(fixes problem caused by implicit include of ../Makefile.inc in certain
submake conditions triggered by makefiles not yet in tree)
- removed mkdir of ${RELEASEDIR}/*; rely upon "snap_pre" target of
etc/Makefile to create all the release directories
- renamed RELINSTALL to RELEASE_INSTALL
- renamed FLOPPYINSTDIR to FLOPPY_RELEASEDIR
- renamed MDSETDIR to MDSET_RELEASEDIR
- removed ITARGET
- move release target from top level to appropriate subdirectory
- ensure release target has correct depends
- replace miniroot's IMAGE_MD_POST with common/Makefile.image IMAGEPOSTBUILD
- Makefile.image: add realall: ${IMAGE}
