Using board, OrangePI Zero3, I'm running a custom dt file sun50i-h616-orangepi-zero3-custom.dts.
I've set the BR2_LINUX_KERNEL_CUSTOM_DTS_PATH in the buildroot menuconfig and dtb succesfully compiles.
I'm also able to successfully manually boot this dtb from uboot. Otherwise I get the error at boot Failed to load '/boot/allwinner/sun50i-h616-orangepi-zero3.dtb' (as this file shouldnt be present).