Allow variants to define an initVariant() function that is called at startup.

See #2080

cores/arduino/main.cpp

@@ -19,10 +19,17 @@
 
 #include <Arduino.h>
 
+// Weak empty variant initialization function.
+// May be redefined by variant files.
+void initVariant() __attribute__((weak));
+void initVariant() { }
+
 int main(void)
 {
 	init();
 
+	initVariant();
+
 #if defined(USBCON)
 	USBDevice.attach();
 #endif