Kudos to Eugenio.
This is how you can use prebuilt dtb/dtbo in the porting process. Add this to the end of debian/rules:
out/KERNEL_OBJ/dtb-merged:
cp -v $(CURDIR)/oneplus-nord-n10.dtb $(KERNEL_OUT)/dtb-merged
out/KERNEL_OBJ/dtbo.img:
cp -v $(CURDIR)/prebuilt_dtbo.img $(KERNEL_OUT)/dtbo.img
This assumes that oneplus-nord-n10.dtb and prebuilt_dtbo.img are on the root directory of the kernel (i.e. the one where debian/ is, not inside). Rename source file according to the name of your device.