add a CPEExportStage

This CL adds a CPEExportStage to all board-specific builders, which
exports the CPE of build packages and uploads this in json format, along
with uploading the stderr of the export tool.

BUG=chromium:341569
TEST=lumpy-paladin and x86-mario-nowithdebug-paladin tryjobs.

Change-Id: I4e8ca7e3bd719c0b34f59769b8c915de4c94baa2
Reviewed-on: https://chromium-review.googlesource.com/190410
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 54d9584..091dbbe 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -471,6 +471,7 @@
         [stages.UploadPrebuiltsStage, board, archive_stage],
         [stages.DevInstallerPrebuiltsStage, board, archive_stage],
         [stages.DebugSymbolsStage, board],
+        [stages.CPEExportStage, board],
     ]
 
     # We can not run hw tests without archiving the payloads.