Fix Serial.flush() blocking if called immediately after Serial.begin().

cores/arduino/SERCOM.cpp

@@ -104,6 +104,8 @@ void SERCOM::enableUART()
 
 void SERCOM::flushUART()
 {
+  // Skip checking transmission completion if data register is empty
+  if(isDataRegisterEmptyUART()) return;
   // Wait for transmission to complete
   while(!sercom->USART.INTFLAG.bit.TXC);
 }