cros_bundle_firmware: Deal with missing skeleton file

This file is optional, so don't require it.

BUG=none
TEST=FEATURES=test sudo -E emerge cros-devutils

Change-Id: Ib73844957aae1f34d9864a7138ced6fd8e28c4e4
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/43001
Reviewed-by: Randall Spangler <rspangler@chromium.org>
diff --git a/host/lib/bundle_firmware.py b/host/lib/bundle_firmware.py
index b4420be..c411fc0 100644
--- a/host/lib/bundle_firmware.py
+++ b/host/lib/bundle_firmware.py
@@ -1008,7 +1008,8 @@
 
     # Get all our blobs ready
     pack.AddProperty('boot', self.uboot_fname)
-    pack.AddProperty('skeleton', self.skeleton_fname)
+    if self.skeleton_fname:
+      pack.AddProperty('skeleton', self.skeleton_fname)
     pack.AddProperty('dtb', fdt.fname)
 
     # Let's create some copies of the fdt for vboot. These can be used to