cros_bundle_firmware: Make u-boot and bmpblk optional
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 <57969107b634cf11cb64e9d768fd46929066adab@chromium.org>
Tested-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,coreos/dev-util