mediatek: fix append-gl-metadata when running in buildbot

Use same logic as in append-metadata so build doesn't fail in case of
missing build-key (it was previously failing on the buildbot runners).

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

target/linux/mediatek/image/filogic.mk

@@ -58,9 +58,12 @@ metadata_gl_json = \
 
 define Build/append-gl-metadata
 	$(if $(SUPPORTED_DEVICES),-echo $(call metadata_gl_json,$(SUPPORTED_DEVICES)) | fwtool -I - $@)
-	[ ! -s "$(BUILD_KEY)" -o ! -s "$@" ] || { \
+	sha256sum "$@" | cut -d" " -f1 > "$@.sha256sum"
+	[ ! -s "$(BUILD_KEY)" -o ! -s "$(BUILD_KEY).ucert" -o ! -s "$@" ] || { \
+		cp "$(BUILD_KEY).ucert" "$@.ucert" ;\
 		usign -S -m "$@" -s "$(BUILD_KEY)" -x "$@.sig" ;\
-		fwtool -S "$@.sig" "$@" ;\
+		ucert -A -c "$@.ucert" -x "$@.sig" ;\
+		fwtool -S "$@.ucert" "$@" ;\
 	}
 endef