rmt_tx_wait_all_done() function

- ESP_OK: Flush transactions successfully - ESP_ERR_INVALID_ARG: Flush transactions failed because of invalid argument - ESP_ERR_TIMEOUT: Flush transactions failed because of timeout - ESP_FAIL: Flush transactions failed because of other error

This function will block forever if the pending transaction can't be finished within a limited time (e.g. an infinite loop transaction). See also `rmt_disable()` for how to terminate a working channel.