cbuildbot_commands.py: call sync_package_status in refresh-packages builder.

Adding calls to sync_package_status at the end of the refresh-packages stage.
This will automatically synchronize tracker issues for outdated packages in
the package spreadsheet, but only for groups that have opted in.  Adding more
groups is simply a matter of making a minor change to cbuildbot_commands.py.

Refactored a couple locations that had hard-coded paths to "chromite/bin" to
use new entry in constants.py.

BUG=chromium-os:25674
TEST=`sync_package_status --pretend --owner=mtennant` behaves normally
TEST=pylint of all files
TEST=`python sync_package_status_unittest.py` passes
TEST=trybot of refresh-packages passes and log shows results of
  sync_package_status run with --pretend.

Change-Id: Iebc8242e7ef4fff5b61afc8232a464dd291d0146
Reviewed-on: https://gerrit.chromium.org/gerrit/17279
Tested-by: Matt Tennant <mtennant@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Matt Tennant <mtennant@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index e595069..83f3e3e 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -24,8 +24,7 @@
 import portage
 
 
-EMERGE_CMD = os.path.join(
-    constants.SOURCE_ROOT, 'chromite/bin/parallel_emerge')
+EMERGE_CMD = os.path.join(constants.CHROMITE_BIN_DIR, 'parallel_emerge')
 CROS_OVERLAY_LIST_CMD = os.path.join(
     constants.SOURCE_ROOT, 'src/platform/dev/host/cros_overlay_list')
 PACKAGE_STABLE = '[stable]'