ramips: add build recipe for senao header

Add it to the Makefile to allow a board which will be supported soon
to use the recipe as well.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/image/Makefile

@@ -78,6 +78,11 @@ define Build/umedia-header
 	fix-u-media-header -T 0x46 -B $(1) -i $@ -o $@.new && mv $@.new $@
 endef
 
+define Build/senao-header
+	$(STAGING_DIR_HOST)/bin/mksenaofw $(1) -e $@ -o $@.new
+	mv $@.new $@
+endef
+
 #
 # The real magic happens inside these templates
 #