How do I Enable Serial Interface on Rpi3?

I’m trying to add an LED to my RPi3 according to this guide:

In order for the LED to work, the serial interface needs to be enabled, but all the guides use raspi-config which us not available on retroarch as far as I can tell.

I poked around via SSH but I didn’t see any config files that would do the trick.

Is this possible with retroarch/lakka? Should I connect the LED another way?