add vanilla_image option to build_chromeos.py

This adds vanilla_iamge option to build_chromeos.py.
With this option, it runs build_packages with --usepkg.
In the test_toolchains.py, it uses this option when
building vanilla image.

BUG=None
TEST=None

Change-Id: I54f85e60ccae833ab6682476e19365a51df5ef29
Reviewed-on: https://gerrit-int.chromium.org/43088
Reviewed-by: Luis Lozano <llozano@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@google.com>
Tested-by: Yunlian Jiang <yunlian@google.com>
diff --git a/test_toolchains.py b/test_toolchains.py
index 18448ab..702af56 100644
--- a/test_toolchains.py
+++ b/test_toolchains.py
@@ -46,6 +46,8 @@
                              "--rebuild"]
       if self._public:
         build_chromeos_args.append("--env=USE=-chrome_internal")
+      if label == "vanilla":
+        build_chromeos_args.append("--vanilla_image")
       ret = build_chromeos.Main(build_chromeos_args)
       if ret:
         raise Exception("Couldn't build ChromeOS!")