Relay integration

[pfeerick]

is this the "3v Arduino Nano"   http://goo.gl/oHtVZx   ?

Unfortunately, not it isn't. That is a 5v Nano (5V16M), the 3 that comes up in the title is in reference to the 3rd hardware revision. (i.e V3.0) 

The 3.3v (which is typically shortened to 3v) Nano seems to be nearly impossible to get hold of now... I have two of them in service, and at the time they were almost as common as the 5v part, but they just don't seem to want to be found now. Having said that, the Pro mini is much more readily available, and is also cheaper than a Nano is. Only downside is that it doesn't have a USB port on it to program with - it has a FTDI-style header on the end that you are intended to connect your USB to serial connector to. Handily enough, the pine64 should be able to connect to it using one of it's own serial headers, so that shouldn't be a problem. So you could go for of these 3.3V Arduno Pro mini clones at a whopping US$2. 

Since the only important thing here is to make sure the micro-controller controlling the relay uses 3.3v logic so you don't need to add logic level/voltage level translation complexity to the link between the relay control board and the pine64, another good option is a Maple Mini. This can be programmed via the Arduino IDE just like other Arduino boards after installing the board support package. They have a lot more memory and run a *lot* faster than the Arduino Uno/Nano boards. You can pick one up for just over US$4.

If you're willing to spend a little more money, the Teensy-LC board that Paul over at PJRC makes is a really, really nice 3.3v Arduino compatible development board. It is slightly overkill for a garage door opener, but will ensure you have plenty of memory to add more features to the project, and is a very well supported board. 

At $42 + shipping from element14, the Gertboard is a nice board (although I prefer the Gertduino, as that gives you the flexibility of Raspberry Pi/pine64 + Arduino + any of the available Arduino shields). However, since all it seems you need from what you have said so far is some isolation from the pine64 to prevent from false triggering the Relay, a Pro Mini/Maple Mini/Teensy-LC is all you need since you already have the relay circuit working with the pine64 on it's own. 

Personally, I would go for the Maple Mine... I have used and will continue to use Arduino Nano/Pro Mini/bare Atmega328P chips in projects and designs in the future, but I think it is the simplest at best fit for your project. Plus you will have plenty of space and features on the board to add all sorts of other cool stuff later if you wish (led lighting, indicator lights, a second control system, etc). 

Hope that doesn't scare you off with information overload!