cbuildbot: Add --workspace command line argument.

Add a new command line argument for defining the "workspace"
directory, and wire it into the firmwarebranch builder class.

BUG=chromium:855291
TEST=run_tests

Change-Id: I2c3e90997262adfcec8ac614ff1e42a12f398a02
Reviewed-on: https://chromium-review.googlesource.com/1121618
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Jason Clinton <jclinton@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index b7a416a..df496ad 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -249,6 +249,9 @@
                          'where the build occurs. For external build configs, '
                          "defaults to 'trybot' directory at top level of your "
                          'repo-managed checkout.')
+  parser.add_option('--workspace', type='path',
+                    api=constants.REEXEC_API_WORKSPACE,
+                    help='Root directory for a secondary checkout .')
   parser.add_option('--bootstrap-dir', type='path',
                     help='Bootstrapping cbuildbot may involve checking out '
                          'multiple copies of chromite. All these checkouts '