Add support for 'brillo' as an entry command distinct from 'cros'. It
still does exactly the same thing... for now.

BUG=brillo:162
TEST=Ran "brillo help" successfully.

Review URL: https://codereview.chromium.org/896393003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294021 0039d316-1c4b-4281-b951-d872f2087c98