Which image are you using in your Galileo? I have the latest image that you can find in Galileo Software Downloads and I do have an interfaces file.
If you're using another image you can use ConnMan to mange your network, you can find more info about it here: https://wiki.archlinux.org/index.php/Connman
Also you can check this threads out, they're about related subjects, maybe they help you:
This one is from Edison but maybe it can work on Galileo as well:
it have not interfaces ,so i don't konw how to change.
With this mirror because he has opencv, g ++; arduino.
I would not build one myself galileo system