increase device object api version to 2 (invalidation of knx flash data stored by older versions)

neccessary due to changes to TableObject.save and restore for router (0x091A) support

src/knx/device_object.h

@@ -9,7 +9,7 @@ class DeviceObject: public InterfaceObject
 public:
     // increase this version anytime DeviceObject-API changes 
     // the following value represents the serialized representation of DeviceObject.
-    const uint16_t apiVersion = 1;
+    const uint16_t apiVersion = 2;
     
     DeviceObject();
     uint8_t* save(uint8_t* buffer) override;