[ci] Whitelist submodule dev branches to enable running submodule-commit checks on dev

This works by checking the $CIRCLE_BRANCH env-var, which is set automatically by
CircleCI. Annoyingly, this is set to the name of the source branch for a merge,
rather than the target branch. The behavior of this check for each branch type
is listed:

if CIRCLE_BRANCH == "master":
  This CI run is the nightly CI run for the master branch.
  Make sure all submodules of the master branch point to master
else if CIRCLE_BRANCH == "dev":
  This CI run is most likely somebody doing a dev->master merge.
  Make sure all submodules of the dev branch point to master
else:
  This CI run is likely somebody merging a feature branch into dev.
  Allow submodule pointers of the dev branch to point to master OR dev

.circleci/config.yml

@@ -622,6 +622,8 @@ workflows:
             # Build verilator
             - install-verilator
 
+            - commit-on-master-check
+
             # Build extra tests
             - build-extra-tests:
                 requires: