Support source upgrades from at least 6.0-RELEASE.

Reviewed by:	imp, obrien

Makefile.inc1

@@ -841,6 +841,10 @@ update:
 # legacy: Build compatibility shims for the next three targets
 #
 legacy:
+.if ${BOOTSTRAPPING} < 600034 && ${BOOTSTRAPPING} != 0
+	@echo "ERROR: Source upgrades from versions prior to 6.0 not supported."; \
+	false
+.endif
 .for _tool in tools/build
 	${_+_}@${ECHODIR} "===> ${_tool} (obj,includes,depend,all,install)"; \
 	    cd ${.CURDIR}/${_tool}; \

UPDATING

@@ -22,13 +22,10 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 8.x IS SLOW:
 	to maximize performance.  (To disable malloc debugging, run
 	ln -s aj /etc/malloc.conf.)
 
-20080121:
+20080123:
 	To upgrade to -current after this date, you must be running
-	FreeBSD after the RELENG_6 branch point.  This means any of the
-	6.x releases are supported and should work.  Upgrading to current
+	FreeBSD not older than 6.0-RELEASE.  Upgrading to -current
 	from 5.x now requires a stop over at RELENG_6 or RELENG_7 systems.
-	FreeBSD_version must be 600029 or higher, although this isn't
-	enforced in the Makefile at this time.
 
 20071128:
 	The ADAPTIVE_GIANT kernel option has been retired because its