I am using beaglebone black and I have one cape with a second ethernet but to have this cape working, I have to apply a dts overlay for the cape before uboot start. The cape and the overlay is working under buildroot but I didn't find a way to do something similar with openWrt buildroot tree.
I can install openwrt with no problem but not include the second ethernet.
I did what you said and it worked.
The challenge did was how to transform the dts overlay in a dtsi include file.
Initially I thought the include should be applied to uboot package but in reality it was applied to the Kernel (what is far better).