Run mm-mtree with whatever -j value the build is using.

Reviewed by:	gjb
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D12807

release/Makefile

@@ -132,7 +132,8 @@ base.txz:
 	cd ${WORLDDIR} && ${IMAKE} distributeworld DISTDIR=${.OBJDIR}/${DISTDIR}
 # Set up mergemaster root database
 	sh ${.CURDIR}/scripts/mm-mtree.sh -m ${WORLDDIR} -F \
-	    "TARGET_ARCH=${TARGET_ARCH} TARGET=${TARGET}" -D "${.OBJDIR}/${DISTDIR}/base"
+	    "TARGET_ARCH=${TARGET_ARCH} TARGET=${TARGET} ${.MAKE.JOBS:D-j${.MAKE.JOBS}}" \
+	    -D "${.OBJDIR}/${DISTDIR}/base"
 	etcupdate extract -B -M "TARGET_ARCH=${TARGET_ARCH} TARGET=${TARGET}" \
 	    -s ${WORLDDIR} -d "${.OBJDIR}/${DISTDIR}/base/var/db/etcupdate"
 # Package all components