check for empty tarball var.

Change-Id: I62a75f75594437a55b12b56617517b4db50ae9c2

Makefile.fetch

@@ -53,6 +53,7 @@ endef
 
 # fetch_Download_item url variable-name
 define fetch_Download_item
+$(if $(strip $($(2))),,$(error fetch__Download_item: $(2) is empty))
 $(if $(filter undefined,$(origin $(subst _TARBALL,_MD5SUM,$(subst _PACK,_MD5SUM,$(2))))),\
 	$(if $(call fetch_Download__is_checksum,$(firstword $(subst -, ,$($(2))))),\
 		$(call fetch__Download_item,$1,$($2),$(firstword $(subst -, ,$($(2))))),\