Add serial method so that lower half driver can provide RX flow control...

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;
 }