Process an internal gateway ready message.

def handle_gateway_ready_20(msg):
    """Process an internal gateway ready message."""
    _LOGGER.info(
        'n:%s c:%s t:%s s:%s p:%s', msg.node_id, msg.child_id, msg.type,
        msg.sub_type, msg.payload)
    msg.gateway.alert(msg)
    return msg.copy(
        node_id=255, ack=0,
        sub_type=msg.gateway.const.Internal.I_DISCOVER, payload='')