Add Uart::availableForWrite()

cores/arduino/Uart.cpp

@@ -77,6 +77,11 @@ int Uart::available()
   return rxBuffer.available();
 }
 
+int Uart::availableForWrite()
+{
+  return (sercom->isDataRegisterEmptyUART() ? 1 : 0);
+}
+
 int Uart::peek()
 {
   return rxBuffer.peek();

cores/arduino/Uart.h

@@ -32,6 +32,7 @@ class Uart : public HardwareSerial
     void begin(unsigned long baudrate, uint16_t config);
     void end();
     int available();
+    int availableForWrite();
     int peek();
     int read();
     void flush();