cros_bundle_firmware: Make u-boot and bmpblk optional

These two parameters are not always required, so can be made optional.
This change is required by the new cros_write_firmware which doesn't
actually have a U-Boot or bmpblk.

BUG=chromium-os:17298
TEST=run new cros_write_firmware, check that it runs OK without these parameters

Change-Id: I9c525463c08b0e7eb28ff767041a9a29c0355146
Reviewed-on: http://gerrit.chromium.org/gerrit/5590
Reviewed-by: Che-Liang Chiou <clchiou@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/host/lib/bundle_firmware.py b/host/lib/bundle_firmware.py
index f79289f..1675a07 100644
--- a/host/lib/bundle_firmware.py
+++ b/host/lib/bundle_firmware.py
@@ -102,7 +102,7 @@
     """
     self._keydir = keydir
 
-  def SetFiles(self, board, uboot, bct, bmpblk):
+  def SetFiles(self, board, bct, uboot=None, bmpblk=None):
     """Set up files required for Bundle.
 
     Args: