Add serial method so that lower half driver can provide RX flow control information. From Jussi Kivilinna

drivers/serial/serial.c

@@ -763,6 +763,17 @@ static ssize_t uart_read(FAR struct file *filep, FAR char *buffer, size_t buflen
         }
     }
 
+#ifdef CONFIG_SERIAL_IFLOWCONTROL
+  if (dev->recv.head == dev->recv.tail)
+    {
+      /* We might leave Rx interrupt disabled if full recv buffer was read
+       * empty. Enable Rx interrupt to make sure that more input is received.
+       */
+
+      uart_enablerxint(dev);
+    }
+#endif
+
   uart_givesem(&dev->recv.sem);
   return recvd;
 }