build system: don't try to add cbfs-files with no backing file

Change-Id: Idd05a552762be92d0d93b357b96442b25a614757
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/12537
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/Makefile.inc b/Makefile.inc
index 10b3368..36977e0 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -225,9 +225,11 @@
 cbfs-files-handler= \
 		$(eval tmp-cbfs-method:=$(word 2, $(subst :, ,$($(2)-file)))) \
 		$(eval $(2)-file:=$(call strip_quotes,$(word 1, $(subst :, ,$($(2)-file))))) \
-		$(if $(wildcard $(1)$($(2)-file)), \
-			$(eval tmp-cbfs-file:= $(wildcard $(1)$($(2)-file))), \
-			$(eval tmp-cbfs-file:= $($(2)-file))) \
+		$(eval tmp-cbfs-file:= ) \
+		$(if $($(2)-file), \
+			$(if $(wildcard $(1)$($(2)-file)), \
+				$(eval tmp-cbfs-file:= $(wildcard $(1)$($(2)-file))), \
+				$(eval tmp-cbfs-file:= $($(2)-file)))) \
 		$(if $(strip $($(2)-required)), \
 			$(if $(wildcard $(tmp-cbfs-file)),, \
 				$(info This build configuration requires $($(2)-required)) \
@@ -247,7 +249,8 @@
 			$(eval tmp-old-cbfs-file:=$(tmp-cbfs-file)) \
 			$(eval tmp-cbfs-file:=$(shell mkdir -p $(obj)/mainboard/$(MAINBOARDDIR); mktemp $(obj)/mainboard/$(MAINBOARDDIR)/cbfs-file.XXXXXX).out) \
 			$(call cbfs-files-processor-$(tmp-cbfs-method),$(tmp-old-cbfs-file),$(tmp-cbfs-file))) \
-		$(eval cbfs-files-$(_cbfs-bucket) += $(tmp-cbfs-file)|$(2)|$($(2)-type)|$($(2)-compression)|$(strip $($(2)-position))|$($(2)-align))\
+		$(if $(tmp-cbfs-file), \
+			$(eval cbfs-files-$(_cbfs-bucket) += $(tmp-cbfs-file)|$(2)|$($(2)-type)|$($(2)-compression)|$(strip $($(2)-position))|$($(2)-align))) \
 		$(eval $(2)-name:=) \
 		$(eval $(2)-type:=) \
 		$(eval $(2)-compression:=) \