Currently, when handling the HCI_STATE_INITIALIZING state in hci_run, there is a test to check if hci_stack->config is NULL, otherwise it is assumed to be an hci_uart_config_t struct. This doesn't allow other transport layers (USB based transports, for example) do define their own configs. Maybe the first fields in the config struct could be a "type" field, which will be mandatory and could used to determine if its UART based when needed.