Add "live-image" and "install-image" target support to build.sh.

"live-image" target builds pre-installed disk images that can be used on
emulators or boot from USB memory sticks to try NetBSD without installation.
Currently amd64, i386, pmax, sparc, sparc64, sun2, sun3, and vax
(which have working emulators and don't require extra tools like preparing
msdosfs or partitioning MD label structures) support this target.

"install-image" target builds an bootable installation disk image that can
be used as an install CD but burned into USB memory sticks etc.
Current only amd64 and i386 (which would support USB boot) have this target.

For more details (and known issue) see following posts on netbsd-bugs@:
http://mail-index.NetBSD.org/netbsd-bugs/2011/08/06/msg023639.html
http://mail-index.NetBSD.org/netbsd-bugs/2011/09/23/msg024207.html
http://mail-index.NetBSD.org/netbsd-bugs/2011/12/07/msg025166.html
http://mail-index.NetBSD.org/netbsd-bugs/2011/12/08/msg025178.html

No particular comments about implementation, and
"go ahead" comments from mrg@, riz@ and christos@.

Closes PR toolchain/45153 and PR misc/45155.

branches: 1.9.12; 1.9.36;
Add an iso_image target (iso-image as a target name doesn't play well with
bsd.subdirs.mk) in distrib/makefile, which builds an iso image for $MACHINE
with binary sets, stored in ${RELEASEDIR}/iso. The image is bootable for:
alpha, amd64, cats, i386, pmax, sgimips, sparc, sparc64, sun3, vax.
mac68k/macppc no there yet because of missing feature in makefs.

call iso_image in distrib/ for iso-image in the top Makefile.

branches: 1.8.18;
yet another distrib overhaul:
- 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}

replace _SRC_TOP_ with NETBSDSRCDIR, since the latter is the "public" version

provide MAKESUMS; run makesums with appropriate environment setup
provide RELINSTALL; install release binaries ${NONBINMODE}
improve some comments

for the moment, use ${_SRC_TOP_}/sys/conf instead of ${KERNSRCDIR}/conf to
find osrelease.sh, since the former should always be available

provide check_DESTDIR and check_RELEASEDIR targets in <bsd.own.mk>
and use those rather than local versions

add check_DESTDIR and check_RELEASEDIR targets

set DISTRIBDIR, DISTRIBREV, DISTRIBVER

incase we muck up number of ../'s below