Support custom suffixes on Makefile outputs

This way Make can use different output files for different targets, and
switch between them without having to reparse all the makefiles.

Change-Id: I00001a09d79025772d966f443ab9f130e35f4720
diff --git a/common/variable.go b/common/variable.go
index ae72761..7f8a6e6 100644
--- a/common/variable.go
+++ b/common/variable.go
@@ -52,6 +52,9 @@
 var zeroProductVariables variableProperties
 
 type productVariables struct {
+	// Suffix to add to generated Makefiles
+	Make_suffix *string `json:",omitempty"`
+
 	Platform_sdk_version *int `json:",omitempty"`
 
 	DeviceName        *string   `json:",omitempty"`