esp8266/uart: Properly initialise UART0 RXD pin in uart_config.

Tested and seems to work.  See #1995.

esp8266/uart.c

@@ -19,6 +19,9 @@
 #include "user_interface.h"
 #include "esp_mphal.h"
 
+// seems that this is missing in the Espressif SDK
+#define FUNC_U0RXD 0
+
 #define UART_REPL UART0
 
 // UartDev is defined and initialized in rom code.
@@ -53,7 +56,7 @@ static void ICACHE_FLASH_ATTR uart_config(uint8 uart_no) {
         ETS_UART_INTR_ATTACH(uart0_rx_intr_handler, NULL);
         PIN_PULLUP_DIS(PERIPHS_IO_MUX_U0TXD_U);
         PIN_FUNC_SELECT(PERIPHS_IO_MUX_U0TXD_U, FUNC_U0TXD);
-        PIN_FUNC_SELECT(PERIPHS_IO_MUX_MTDO_U, FUNC_U0RTS);
+        PIN_FUNC_SELECT(PERIPHS_IO_MUX_U0RXD_U, FUNC_U0RXD);
     }
 
     uart_div_modify(uart_no, UART_CLK_FREQ / (UartDev.baut_rate));