image: add support for overriding kernel/rootfs images in sysupgrade-tar template

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

include/image-commands.mk

@@ -151,7 +151,7 @@ endef
 define Build/sysupgrade-tar
 	sh $(TOPDIR)/scripts/sysupgrade-tar.sh \
 		--board $(if $(BOARD_NAME),$(BOARD_NAME),$(DEVICE_NAME)) \
-		--kernel $(word 1,$^) \
-		--rootfs $(word 2,$^) \
+		--kernel $(call param_get_default,kernel,$(1),$(word 1,$^)) \
+		--rootfs $(call param_get_default,rootfs,$(1),$(word 2,$^)) \
 		$@
 endef

include/image.mk

@@ -29,6 +29,7 @@ override NO_TRACE_MAKE:=$(_SINGLE)$(NO_TRACE_MAKE)
 
 target_params = $(subst +,$(space),$*)
 param_get = $(patsubst $(1)=%,%,$(filter $(1)=%,$(2)))
+param_get_default = $(firstword $(call param_get,$(1),$(2)) $(3))
 param_mangle = $(subst $(space),_,$(strip $(1)))
 param_unmangle = $(subst _,$(space),$(1))