Allow configs to pass a custom chromite/bin script to run in lieu of cbuildbot.
Allow configs to pass a custom chromite/bin script to run in lieu of cbuildbot.

BUG=chromium-os:36919
TEST=Ran it with new use in chromeos.chrome.


Review URL: https://chromiumcodereview.appspot.com/11416335

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@171100 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build