Consolidate the nanopb-c and regular protobuf build rules.

Bug: 20093047
Change-Id: I38ebd748aacce6d215403da481ad04833810a995
diff --git a/core/definitions.mk b/core/definitions.mk
index 35e3a79..d05a596 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -957,7 +957,7 @@
 endef
 
 ######################################################################
-## Commands for running protoc to compile .proto into .pb.cc and .pb.h
+## Commands for running protoc to compile .proto into .pb.cc (or.pb.c) and .pb.h
 ######################################################################
 define transform-proto-to-cc
 @echo "Protoc: $@ <= $<"
@@ -965,7 +965,7 @@
 $(hide) $(PROTOC) \
 	$(addprefix --proto_path=, $(PRIVATE_PROTO_INCLUDES)) \
 	$(PRIVATE_PROTOC_FLAGS) \
-	--cpp_out=$(PRIVATE_PROTO_CC_OUTPUT_DIR) $<
+	$<
 endef