Add --sourcemap-sources-root to RN packager

Reviewed By: davidaurelio

Differential Revision: D4357122

fbshipit-source-id: dc19474aa54ea4684a6b1c586a9d1e5da0980327

local-cli/bundle/types.flow.js

@@ -36,6 +36,7 @@ export type OutputOptions = {
   dev?: boolean,
   platform: string,
   sourcemapOutput?: string,
+  sourcemapSourcesRoot?: string,
 };
 
 export type RequestOptions = {|