build: add image command for calling kernel2minor

Signed-off-by: Felix Fietkau <[email protected]>

include/image-commands.mk

@@ -190,3 +190,8 @@ metadata_json = \
 define Build/append-metadata
 	$(if $(SUPPORTED_DEVICES),echo $(call metadata_json,$(SUPPORTED_DEVICES)) | fwtool -I - $@)
 endef
+
+define Build/kernel2minor
+	kernel2minor -k $@ -r [email protected] $(1)
+	mv [email protected] $@
+endef