Applying the patch from Pantelis Antoniou that gives DTC the ability to ...

...define symbols.

This is mentioned often in the Beaglebone Black community for building device tree overlays using the -@ syntax. Here is the original patch:
https://patchwork.kernel.org/patch/1934471/

And here is an article from Adafruit talking about it's use:
https://learn.adafruit.com/introduction-to-the-beaglebone-black-device-tree/compiling-an-overlay

This builds on the Beaglebone Black and works as expected (to the best of my abilities)