Read product variables from soong.variables

Refactor the soong.config loading code to support reading in
product variables from soong.variables.

Change-Id: I389e6bb5c501b53167267d5f5d0d25557811cf72

cmd/soong_build/main.go

@@ -49,5 +49,5 @@ func main() {
 	// Temporary hack
 	//ctx.SetIgnoreUnknownModuleTypes(true)
 
-	bootstrap.Main(ctx, configuration, common.ConfigFileName)
+	bootstrap.Main(ctx, configuration, common.ConfigFileName, common.ProductVariablesFileName)
 }