Use Ecto in app under umbrella

Hi All,
I have problem to generate Repo and use ecto in application under umbrella, because lack of config file.

 mix ecto.gen.repo -r App.Repo
* creating lib/app
* creating lib/app/repo.ex
* updating config/config.exs
** (File.Error) could not write to file "config/config.exs": no such file or directory
    (elixir) lib/file.ex:1050: File.write!/3
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/cli.ex:79: Mix.CLI.run_task/2

And I found that since v1.9 there is no config dir in apps under umbrella.

My question is:
Is there any other way for using ecto in apps?
Elixir version 1.9.4

Thanks in advance!

#project fun from mobile/apps/app/mix.exs
  def project do
      app: :app,
      version: "0.1.0",
      build_path: "../../_build",
      config_path: "../../config/config.exs",
      deps_path: "../../deps",
      lockfile: "../../mix.lock",
      elixir: "~> 1.9",
      start_permanent: Mix.env() == :prod,
      deps: deps()