Allow specifying a build variant

Test: multiproduct_kati --variant userdebug
Change-Id: I6548889caf868cad6270110c038753d029247572
diff --git a/cmd/multiproduct_kati/main.go b/cmd/multiproduct_kati/main.go
index 97d4cfa..b12628e 100644
--- a/cmd/multiproduct_kati/main.go
+++ b/cmd/multiproduct_kati/main.go
@@ -51,6 +51,8 @@
 var onlyConfig = flag.Bool("only-config", false, "Only run product config (not Soong or Kati)")
 var onlySoong = flag.Bool("only-soong", false, "Only run product config and Soong (not Kati)")
 
+var buildVariant = flag.String("variant", "eng", "build variant to use")
+
 type Product struct {
 	ctx    build.Context
 	config build.Config
@@ -225,7 +227,7 @@
 
 			productConfig := build.NewConfig(productCtx)
 			productConfig.Environment().Set("OUT_DIR", productOutDir)
-			productConfig.Lunch(productCtx, product, "eng")
+			productConfig.Lunch(productCtx, product, *buildVariant)
 
 			build.Build(productCtx, productConfig, build.BuildProductConfig)
 			productConfigs <- Product{productCtx, productConfig}