initVariant() is now declared as external "C" function.

See #2080 for more details.

(Filtered from arduino/Arduino@bb095a2)

Author: cmaglie

core/Arduino.h

@@ -113,6 +113,7 @@ typedef uint8_t boolean;
 typedef uint8_t byte;
 
 void init(void);
+void initVariant(void);
 
 void pinMode(uint8_t, uint8_t);
 void digitalWrite(uint8_t, uint8_t);