UART: adapt to namespaced ArduinoAPI

Author: facchinm

cores/arduino/UART.h

@@ -28,6 +28,10 @@
 #include <inttypes.h>
 #include "api/HardwareSerial.h"
 
+#if ARDUINO_API_VERSION > 10000
+using namespace arduino;
+#endif
+
 // Define constants and variables for buffering incoming serial data.  We're
 // using a ring buffer (I think), in which head is the index of the location
 // to which to write the next incoming character and tail is the index of the