oxnas: require image metadata

Now that we generate images with metadata attached, require it in order
to prevent accidentally flashing the wrong image to a board.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/oxnas/base-files/lib/upgrade/platform.sh

@@ -5,6 +5,7 @@
 . /lib/oxnas.sh
 
 RAMFS_COPY_DATA=/lib/oxnas.sh
+REQUIRE_IMAGE_METADATA=1
 
 platform_check_image() {
 	local board=$(oxnas_board_name)