packages: Allow stageworld/stagekernel to run with make jobs.

The -B was originally added in projects/release-pkg r289381 as a copy
of what 'make world' did at the time.  The -B was removed from
the 'installworld' call in 'world' in r303844 though.  The staging
of files is safe to run in parallel.

MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon

Makefile.inc1

@@ -1539,8 +1539,8 @@ real-packages:	stage-packages create-packages sign-packages .PHONY
 stage-packages: .PHONY
 	@mkdir -p ${REPODIR} ${WSTAGEDIR} ${KSTAGEDIR}
 	${_+_}@cd ${.CURDIR}; \
-		${MAKE} DESTDIR=${WSTAGEDIR} -DNO_ROOT -B stageworld ; \
-		${MAKE} DESTDIR=${KSTAGEDIR} -DNO_ROOT -B stagekernel
+		${MAKE} DESTDIR=${WSTAGEDIR} -DNO_ROOT stageworld; \
+		${MAKE} DESTDIR=${KSTAGEDIR} -DNO_ROOT stagekernel
 
 create-packages:	_pkgbootstrap .PHONY
 	@mkdir -p ${REPODIR}