build: add template for installing device .dtb files

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/image-commands.mk

@@ -46,6 +46,14 @@ define Build/append-dtb
 	cat $@.dtb >> $@
 endef
 
+define Build/install-dtb
+	$(foreach dts,$(DEVICE_DTS), \
+		$(CP) \
+			$(DTS_DIR)/$(dts).dtb \
+			$(BIN_DIR)/$(IMG_PREFIX)-$(dts).dtb; \
+	)
+endef
+
 define Build/fit
 	$(TOPDIR)/scripts/mkits.sh \
 		-D $(DEVICE_NAME) -o $@.its -k $@ \