base-files: uci-defaults: do config flush in one shot

Moving a file between tmpfs and other fs is neither
faster nor safer, thus no point in doing it in two steps.
Use new jshn option to write output directly to file.

Originally discussed here:
http://lists.openwrt.org/pipermail/openwrt-devel/2017-December/010127.html

Signed-off-by: Roman Yeryomin <roman@advem.lv>

package/base-files/files/lib/functions/uci-defaults.sh

@@ -615,6 +615,5 @@ board_config_update() {
 }
 
 board_config_flush() {
-	json_dump -i > /tmp/.board.json
-	mv /tmp/.board.json ${CFG}
+	json_dump -i -o ${CFG}
 }