GPL requires source code be offered for any software distributed. Changes to the build system probably aren't covered, but a modified DTB probably is. The final image also has the Linux kernel and other GPL components, so that definitely is.
Bottom line is that GPL is supposed to be permissive. DTS is easily gotten from DTB. And you're probably good as long as you're not being a jerk about it.
If that bothers you, you could also make a overlay (dtbo) and just patch the DTB on boot. You'd avoid the custom image and also make it more clear that your DTB is tweaked.
Clark McCauley reacted to tparys in Clarification on GPL 2.0 License Applicability