metadata: fixup "board"

currently the "board" parameter contains $(BOARD) which actually results
to `<target>` (like ramips, ar71xx) without subtarget. However, one
actually excepts (not?) to contain BOARD_NAME or DEVICE_NAME.

Signed-off-by: Paul Spooren <mail@aparcar.org>

include/image-commands.mk

@@ -360,7 +360,7 @@ metadata_json = \
 			"dist": "$(call json_quote,$(VERSION_DIST))", \
 			"version": "$(call json_quote,$(VERSION_NUMBER))", \
 			"revision": "$(call json_quote,$(REVISION))", \
-			"board": "$(call json_quote,$(BOARD))" \
+			"board": "$(call json_quote,$(if $(BOARD_NAME),$(BOARD_NAME),$(DEVICE_NAME)))" \
 		} \
 	}'