Refactor SDK tarball creation to only happen once, in a dedicated stage

This eliminates an unnecessary duplicate tarball creation inside upload_prebuilts.py
that prolonged the run unecessarily and made testing harder.

BUG=chromium-os:29624
TEST=trybot chromiumos-sdk
TEST=unit tests

Change-Id: I046e3861649012f4c8db1fe6eb35eb8f79e5aae8
Reviewed-on: https://gerrit.chromium.org/gerrit/32024
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Zdenek Behan <zbehan@chromium.org>
Tested-by: Zdenek Behan <zbehan@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index ebdc0fa..b4a45c7 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -401,6 +401,7 @@
 
     # TODO(sosa): Split these out into classes.
     if self.build_config['build_type'] == constants.CHROOT_BUILDER_TYPE:
+      self._RunStage(stages.SDKPackageStage)
       self._RunStage(stages.SDKTestStage)
       self._RunStage(stages.UploadPrebuiltsStage,
                      constants.CHROOT_BUILDER_BOARD, None)