STM8 uart.c: Return mutex to allow multiple threads access to the UART.

ports/stm8/uart.c

@@ -67,7 +67,10 @@ char putchar (char c)
         /* Loop until the end of transmission */
         while (UART2_GetFlagStatus(UART2_FLAG_TXE) == RESET)
             ;
-
+
+        /* Return mutex access */
+        atomMutexPut(&uart_mutex);
+
     }
 
     return (c);