image: use ucert to append signature

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

include/image-commands.mk

@@ -309,6 +309,12 @@ metadata_json = \
 
 define Build/append-metadata
 	$(if $(SUPPORTED_DEVICES),-echo $(call metadata_json,$(SUPPORTED_DEVICES)) | fwtool -I - $@)
+	[ -s "$(BUILD_KEY)" -a -s "$(BUILD_KEY).ucert" ] && { \
+		cp "$(BUILD_KEY).ucert" "$@.ucert" ;\
+		usign -S -m "$@" -s "$(BUILD_KEY)" -x "$@.sig" ;\
+		ucert -A -c "$@.ucert" -x "$@.sig" ;\
+		fwtool -S "$@.ucert" "$@" ;\
+	}
 endef
 
 define Build/kernel2minor