esp8266: Add uart_rx_wait and uart_rx_char functions.

esp8266/uart.c

@@ -222,6 +222,26 @@ int uart0_rx(void) {
 }
 */
 
+// Waits at most timeout microseconds for at least 1 char to become ready for reading.
+// Returns true if something available, false if not.
+bool uart_rx_wait(uint32_t timeout_us) {
+    uint32_t start = system_get_time();
+    for (;;) {
+        if (input_buf.iget != input_buf.iput) {
+            return true; // have at least 1 char ready for reading
+        }
+        if (system_get_time() - start >= timeout_us) {
+            return false; // timeout
+        }
+        ets_event_poll();
+    }
+}
+
+// Returns char from the input buffer, else -1 if buffer is empty.
+int uart_rx_char(void) {
+    return ringbuf_get(&input_buf);
+}
+
 int uart_rx_one_char(uint8 uart_no) {
     if (READ_PERI_REG(UART_STATUS(uart_no)) & (UART_RXFIFO_CNT << UART_RXFIFO_CNT_S)) {
         return READ_PERI_REG(UART_FIFO(uart_no)) & 0xff;