cbuildbot: run uprev for sdk builders

With the toolchain packages being cros-workon now, we want them to uprev
in the sdk builder so that we can easily test/validate changes.

BUG=chromium-os:34649
TEST=`cbuildbot chromiumos-sdk` ran uprev stage

Change-Id: Ic69bba3ae125a275d7ffce8e808c4bc32a7a71de
Reviewed-on: https://gerrit.chromium.org/gerrit/38327
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 83cc7a7..1cf5de0 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -380,6 +380,7 @@
     """Runs through build process."""
     # TODO(sosa): Split these out into classes.
     if self.build_config['build_type'] == constants.CHROOT_BUILDER_TYPE:
+      self._RunStage(stages.UprevStage, [])
       self._RunStage(stages.BuildBoardStage, [constants.CHROOT_BUILDER_BOARD])
       self._RunStage(stages.SDKPackageStage)
       self._RunStage(stages.SDKTestStage)