build: add apend-uboot command

This commit adds an append-uboot command to append U-Boot from the
bin-directory.

Signed-off-by: David Bauer <[email protected]>

include/image-commands.mk

@@ -186,6 +186,10 @@ define Build/append-ubi
 	rm [email protected]
 endef
 
+define Build/append-uboot
+	dd if=$(UBOOT_PATH) >> $@
+endef
+
 define Build/pad-to
 	dd if=$@ [email protected] bs=$(1) conv=sync
 	mv [email protected] $@