Added initVariant() hook

as in the other Arduino cores

cores/arduino/main.cpp

 /*
-  Copyright (c) 2014 Arduino.  All right reserved.
+  Copyright (c) 2015 Arduino LLC.  All right reserved.
 
   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
@@ -19,6 +19,11 @@
 #define ARDUINO_MAIN
 #include "Arduino.h"
 
+// Weak empty variant initialization function.
+// May be redefined by variant files.
+void initVariant() __attribute__((weak));
+void initVariant() { }
+
 /*
  * \brief Main entry point of Arduino application
  */
@@ -26,6 +31,8 @@ int main( void )
 {
   init();
 
+  initVariant();
+
   delay(1);
 #if defined(USBCON)
   USBDevice.init();