cros_mark_as_stable: make it work outside of a chroot

In some cases (like the cros-sdk), we don't need (or want) a chroot in
order to uprev packages since we'll be throwing it away.  So tweak the
logic slightly to skip steps that require a chroot if we are run outside
of one.

BUG=None
TEST=`./buildbot/run_tests` passes
TEST=`cbuildbot chromiumos-sdk` uprevs w/out a chroot
TEST=`cbuildbot x86-generic-paladin` uprevs w/a chroot

Change-Id: I2760088d69f30a7be5d6f960cc6bcff66e07b5b5
Reviewed-on: https://gerrit.chromium.org/gerrit/38390
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 659272d..4911be7 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -380,7 +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.UprevStage, boards=[], enter_chroot=False)
       self._RunStage(stages.BuildBoardStage, [constants.CHROOT_BUILDER_BOARD])
       self._RunStage(stages.SDKPackageStage)
       self._RunStage(stages.SDKTestStage)