cros_bundle_firmware: Pass PackFirmware properties back from bundle

Some of the properties created by PackFirmware are useful later when
writing firmware to a device. Return these from BundleFirmware's Start()
method.

BUG=chrome-os-partner:10075
TEST=emerge-tegra2_seaboard chromeos-bootimage

Change-Id: Ic23a3c20e392e4d6ce7c1dfb118e95a660e343bc
Reviewed-on: https://gerrit.chromium.org/gerrit/24762
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Ready: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/host/lib/bundle_firmware.py b/host/lib/bundle_firmware.py
index 1184b28..db93cd5 100644
--- a/host/lib/bundle_firmware.py
+++ b/host/lib/bundle_firmware.py
@@ -816,4 +816,4 @@
     if output_fname:
       shutil.copyfile(image, output_fname)
       self._out.Notice("Output image '%s'" % output_fname)
-    return image
+    return image, pack.props