Add a 'make cleanuniverse'.

This will remove all build files for the source directory
when MK_UNIFIED_OBJDIR is enabled.

Sponsored by:	Dell EMC Isilon

Makefile

@@ -122,6 +122,7 @@
 TGTS=	all all-man buildenv buildenvvars buildkernel buildworld \
 	check check-old check-old-dirs check-old-files check-old-libs \
 	checkdpadd checkworld clean cleandepend cleandir cleanworld \
+	cleanuniverse \
 	delete-old delete-old-dirs delete-old-files delete-old-libs \
 	depend distribute distributekernel distributekernel.debug \
 	distributeworld distrib-dirs distribution doxygen \

Makefile.inc1

@@ -2751,14 +2751,21 @@ builddtb: .PHONY
 # in this makefile.  We don't do a cleandir walk if MK_AUTO_OBJ is yes
 # since it is not possible for files to land in the wrong place.
 #
-BW_CANONICALOBJDIR:=${OBJTOP}
-cleanworld: .PHONY
-.if exists(${BW_CANONICALOBJDIR}/)
-	-rm -rf ${BW_CANONICALOBJDIR}/*
+.if make(cleanworld)
+BW_CANONICALOBJDIR:=${OBJTOP}/
+.elif make(cleanuniverse)
+BW_CANONICALOBJDIR:=${OBJROOT}
+.if ${MK_UNIFIED_OBJDIR} == "no"
+.error ${.TARGETS} only supported with WITH_UNIFIED_OBJDIR enabled.
+.endif
+.endif
+cleanworld cleanuniverse: .PHONY
+.if !empty(BW_CANONICALOBJDIR) && exists(${BW_CANONICALOBJDIR})
+	-rm -rf ${BW_CANONICALOBJDIR}*
 	-chflags -R 0 ${BW_CANONICALOBJDIR}
-	rm -rf ${BW_CANONICALOBJDIR}/*
+	rm -rf ${BW_CANONICALOBJDIR}*
 .endif
-.if ${MK_AUTO_OBJ} == "no"
+.if make(cleanworld) && ${MK_AUTO_OBJ} == "no"
 .if ${.CURDIR} == ${.OBJDIR} || ${.CURDIR}/obj == ${.OBJDIR}
 	#   To be safe in this case, fall back to a 'make cleandir'
 	${_+_}@cd ${.CURDIR}; ${MAKE} cleandir

share/man/man7/build.7

@@ -207,8 +207,15 @@ for cross builds and native builds with variable
 set.
 .It Cm cleanworld
 Attempt to clean up targets built by a preceding
-.Cm buildworld
-step.
+.Cm buildworld ,
+or similar step built from this source directory.
+.It Cm cleanuniverse
+When
+.Va WITH_UNIFIED_OBJDIR
+is enabled, attempt to clean up targets built by a preceding
+.Cm buildworld ,
+.Cm universe ,
+or similar step, for any architecture built from this source directory.
 .It Cm distributeworld
 Distribute everything compiled by a preceding
 .Cm buildworld