Adding 'realclean' target to /usr/src/Makefile

[M. Warner Losh]

In message: <20040215092056.GF22136 at saboteur.dek.spc.org>
            Bruce M Simpson <bms at spc.org> writes:
: It would be helpful if it were pointed out in documentation somewhere
: that the path to the compile and source directories, when doing NFS
: kernel installs, has to be identical to those which were in effect on
^^^^^^^^
: the build box due to the treatment of MAKEOBJDIRPREFIX during the modules
: build (and subsequent modules/* path creation).

Remove the word kernel.  It has to be identical for *ALL* installs.
*OR* you have to play games with symbolic links (which is what Timing
Solutions does for their chroot build environments).

Warner