modbus-rtu: esp-idf port, make sure line endings are ignored on UART

By default some line ending conversion may be done, so make sure to
disable this on the UART used with libmodbus.

src/esp-idf/README.md

@@ -64,3 +64,7 @@ and would require a heavy change in the library interface.
   a standard configuration. Please check the `LWIP_MAX_SOCKETS` configuration
   variable.
 
+- Older versions (<=5.3.1) of ESP-IDF contain a buglet in the serial read code that
+  may cause some packet loss under certain circumstances (when line ending
+  chars are present), see also: https://github.com/espressif/esp-idf/issues/14155
+  Please use a newer version of ESP-IDF if you see this behavior.

src/modbus-rtu.c

@@ -588,6 +588,8 @@ static int _modbus_rtu_connect(modbus_t *ctx)
     }
 
     uart_vfs_dev_use_driver(uart_num);
+    uart_vfs_dev_port_set_rx_line_endings(uart_num, ESP_LINE_ENDINGS_LF);
+    uart_vfs_dev_port_set_tx_line_endings(uart_num, ESP_LINE_ENDINGS_LF);
 
     return 0;
 }