Convert freebsd-version to using -v

newvers.sh has supported a variable setting only mode, use that in
preference to grep to future proof this script from changes there.

Differential Revision: https://reviews.freebsd.org/D19849

bin/freebsd-version/Makefile

@@ -7,7 +7,7 @@ CLEANFILES = freebsd-version freebsd-version.sh
 NEWVERS = ${SRCTOP}/sys/conf/newvers.sh
 
 freebsd-version.sh: ${.CURDIR}/freebsd-version.sh.in ${NEWVERS}
-	eval $$(egrep '^(TYPE|REVISION|BRANCH)=' ${NEWVERS}) ; \
+	eval $$(sh ${NEWVERS} -v); \
 	if ! sed -e "\
 			s/@@TYPE@@/$${TYPE}/g; \
 			s/@@REVISION@@/$${REVISION}/g; \