Thank you for the resources, and you're right, I should probably be asking a lawyer.
That said, the thing that's been throwing me off is that the GPL FAQ specifically states that
and uses GCC as an example. Using GCC to build a binary does not make the binary subject to GPL. It seems like there's a similarity here to armbian/build which is a tool that produces an artifact and the code within customize-image.sh is my own code.
This question really came up for me when I was modifying a DTS file to disable a peripheral on my device and was investigating whether that change would need to be open sourced. I found that the parent DTS file that I was patching was licensed under GPL 2.0 OR MIT, so if I'm reading everything correctly, it seems like neither the customize-image.sh script nor my DTS patches need to published and licensed under GPL 2.0.
But I'm posing here in case I've egregiously misunderstood something. If there's nuance and subtlety here, where my assumptions above might be correct, or might not be depending on other factors, then I may need to get an attorney involved.