MAKEALL: Add -s to '${MAKE} tidy' section

When BUILD_NBUILDS is > 1 we run the tidy command.  With the addition of
DocBook this now includes a -C doc/DocBook and a 'entering/leaving' pair
of messages happen.  Since we don't want to see what's being cleaned
here, we can just invoke make -s like we do when building.

Signed-off-by: Tom Rini <trini@ti.com>

MAKEALL

@@ -640,7 +640,7 @@ build_target() {
 	fi
 
 	if [ $BUILD_MANY == 1 ] ; then
-		${MAKE} tidy
+		${MAKE} -s tidy
 
 		if [ -s ${LOG_DIR}/${target}.ERR ] ; then
 			cp ${LOG_DIR}/${target}.ERR ${OUTPUT_PREFIX}/ERR/${target}