cbuildbot: Add 'no-publish-prebuilts-conf' flag

This approach would enable running a production instance of the SDK
builder based on a branch and having that builder upload binary packages
but *not* write any changes to prebuilt.conf or sdk_version.conf. A
person could then manually update the prebuilt.conf paths on the branch
to point to these uploaded artifacts as a way of providing new
cross-compilers on a release branch.

BUG=chromium:1055914
TEST=None

Change-Id: I622af5b2d8a41e7afcb02c86b22a9f07f3b11e6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2072942
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 2a8e37c..e1e7b11 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -475,6 +475,12 @@
                           dest='cbb_snapshot_revision', default=None,
                           help='Snapshot manifest revision to sync to '
                                'for building.')
+  group.add_remote_option(
+      '--no-publish-prebuilt-confs',
+      dest='publish',
+      action='store_false',
+      default=True,
+      help="Don't publish git commits to prebuilt.conf or sdk_version.conf")
 
   parser.add_argument_group(group)