Various fixes for SAMD (#510)

* Only serial GW should hold back on USB

* Fixing some examples on archs other than avr

* HwInit() is needed on samd, to init I2C

examples/SecurityPersonalizer/SecurityPersonalizer.ino

@@ -879,7 +879,7 @@ void setup()
   (void)key;
 
   Serial.begin(115200);
-
+  hwInit();
   Serial.println(F("Personalization sketch for MySensors usage."));
   Serial.println(F("-------------------------------------------"));