buildbot: pgo: cleanup both pgo_generate and pgo_use images.

Currently the buildbot cleans up only the first config in a group of
configurations. The PGO builder runs out of disk because of this. So fix
buildbot to clean up all the configs in a group of configs.

BUG=chromium:256240
TEST=cbuildbot ...

Change-Id: I2ef3c313234406fe14e78b07f9bd5e899104f2ff
Reviewed-on: https://gerrit.chromium.org/gerrit/62853
Reviewed-by: Scott Zawalski <scottz@chromium.org>
Reviewed-by: Peter Mayo <petermayo@chromium.org>
Commit-Queue: asharif <asharif@chromium.org>
Tested-by: asharif <asharif@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index df340fd..a9c75a9 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -382,6 +382,7 @@
     """Run background board-specific stages for the specified board."""
     archive_stage = self.archive_stages[BoardConfig(board, config['name'])]
     if config['pgo_generate']:
+      self._RunParallelStages([archive_stage])
       return
     if compilecheck:
       self._RunStage(stages.BuildPackagesStage, board, archive_stage,