refactor(uart): add support to be able to test LP_UART port

Increase LP_UART_EMPTY_THRESH_DEFAULT value to 4. The original value
could cause the FIFO become empty before filling next data into the FIFO
when the buadrate is high. TX_DONE interrupt would raise before actual
transmission complete in such case.

components/esp_driver_uart/include/driver/uart.h

@@ -1,15 +1,11 @@
 /*
- * SPDX-FileCopyrightText: 2015-2022 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2015-2024 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
 
 #pragma once
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include "esp_err.h"
 #include "esp_intr_alloc.h"
 #include "soc/soc_caps.h"
@@ -17,6 +13,10 @@ extern "C" {
 #include "freertos/queue.h"
 #include "hal/uart_types.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* @brief When calling `uart_set_pin`, instead of GPIO number, `UART_PIN_NO_CHANGE`
  *        can be provided to keep the currently allocated pin.
  */