Use schedule() not fiber_sleep(0) in MicroBitSerial.cpp (Fix #461)

  - MicroBitSerial use wake_on_event for condition synchronisation.
  - By contract, a fiber doing this should subsequently call schedule()
    and no other blocking operation

source/drivers/MicroBitSerial.cpp

@@ -295,7 +295,7 @@ void MicroBitSerial::send(MicroBitSerialMode mode)
         while(txBufferedSize() > 0);
 
     if(mode == SYNC_SLEEP)
-        fiber_sleep(0);
+        schedule();
 }
 
 /**