release: Redirect etcupdate logfile to /dev/null.

Stop shipping a log file for etcupdate. This is a source of
non-reproducability as it uses mktemp thereby guaranteeing the output is
different each run.

Differential Revision:	https://reviews.freebsd.org/D46317

(cherry picked from commit e972e408d19a58c4f2855eface487d06ef73b799)

release/Makefile

@@ -129,7 +129,7 @@ base.txz:
 	sh ${WORLDDIR}/usr.sbin/etcupdate/etcupdate.sh extract -B \
 	    -m "${MAKE}" -M "TARGET_ARCH=${TARGET_ARCH} TARGET=${TARGET}" \
 	    -s ${WORLDDIR} -d "${.OBJDIR}/${DISTDIR}/base/var/db/etcupdate" \
-	    ${NO_ROOT:D-N}
+	    -L /dev/null ${NO_ROOT:D-N}
 .if defined(NO_ROOT)
 	echo "./var/db/etcupdate type=dir uname=root gname=wheel mode=0755" >> ${.OBJDIR}/${DISTDIR}/base.meta
 	sed -n 's,^\.,./var/db/etcupdate/current,p' ${.OBJDIR}/${DISTDIR}/base/var/db/etcupdate/current/METALOG \