fix(esp8266): fix UART output error when start up

Add UART system API to flush UART and wait until all characters are sent.

components/esp8266/source/rom.c

@@ -0,0 +1,33 @@
+// Copyright 2019-2020 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "rom/uart.h"
+
+void uart_tx_wait_idle(uint8_t uart_no)
+{
+    uint32_t tx_bytes;
+    uint32_t baudrate, byte_delay_us;
+    uart_dev_t *const UART[2] = {&uart0, &uart1};
+    uart_dev_t *const uart = UART[uart_no];
+    
+    baudrate = (UART_CLK_FREQ / (uart->clk_div.val & 0xFFFFF));
+    byte_delay_us = (uint32_t)(10000000 / baudrate);
+
+    do {
+        tx_bytes = uart->status.txfifo_cnt;
+        /* either tx count or state is non-zero */
+    } while (tx_bytes);
+
+    ets_delay_us(byte_delay_us);
+}