update support for orangepi_zero_2w

configs/orangepi_zero_2w_defconfig

@@ -38,6 +38,8 @@ export BUSYBOX_INITRD_CONFIG_FILE=$root/board/${BOARD_NAME}/busybox-initrd-${BUS
 export BUSYBOX_LIST_FILE=$root/board/${BOARD_NAME}/busybox-list-${BUSYBOX_VERSION}.config
 export BUSYBOX_INITRD_LIST_FILE=$root/board/${BOARD_NAME}/busybox-initrd-list-${BUSYBOX_VERSION}.config
 
+export ROOTFS_OVERLAY=$root/board/${BOARD_NAME}/rootfs_overlay
+
 export ZLIB_ENABLE=n
 export DROPBEAR_ENABLE=n
 export IMAGE_USABLE_ENABLE=n