Break out prebuilts logic for dev_installer into its own stage.

This breaks out the dev_installer logic from the prebuilts logic
allowing us to globally set prebuilts=False for official builds
and not have that clobber dev_installer uploads from official builds.

BUG=chromium-os:35560
TEST=Unittests + pylint + additional unittests + run through run with
remote trybot with paladin + chrome pfq + canary configs.

Change-Id: Idcdd2524f80e73a844df54bf40bc81b5562fc1d4
Reviewed-on: https://gerrit.chromium.org/gerrit/36247
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Chris Sosa <sosa@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 924000f..8f83a9b 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -361,7 +361,8 @@
                   [stages.ChromeTestStage, board, archive_stage],
                   [stages.SignerTestStage, board, archive_stage],
                   [stages.UnitTestStage, board],
-                  [stages.UploadPrebuiltsStage, board, archive_stage]]
+                  [stages.UploadPrebuiltsStage, board, archive_stage],
+                  [stages.DevInstallerPrebuiltsStage, board, archive_stage]]
 
     # We can not run hw tests without archiving the payloads.
     if self.options.archive: