Yocto add dtbo into boot directory

Question

I'm having an issue with adding a dtbo into the boot sector of a rpi-cm3 yocto image on the dunfell branch. I have followed this post without success.

Here is my procedure:

I have a recipe which inherits from the devicetree class:

DESCRIPTION = "Affinage device overlays for device tree."

inherit devicetree

SRC_URI = "file://affinage-actuator-overlay.dts"

S = "${WORKDIR}"

COMPATIBLE_MACHINE = "raspberrypi-cm3"

This successfully creates the files required for the boot sector:

└── boot
    └── devicetree
        └── affinage-actuator-overlay.dtbo

The recipe doesn't have a FILES_${PN} for installation because the devicetree class has this already within the class code: FILES_${PN} = "/boot/devicetree/*.dtb /boot/devicetree/*.dtbo"

I have added the recipe into my image:

IMAGE_INSTALL += " affinage-devices"

The resultant image builds and the affinage-devices.bb recipe creating the dtbo file but the file never ends up in the /boot/devicetree directory of the actual image.

In the linked post above it states that you should inherit the devicetree class into the kernel recipe (although there are two conflicting posts on this in the thread). When I do inherit in the kernel recipe I get the following error:

    if [ -n "${KERNEL_DTC_FLAGS}" ]; then
                                        ^
TabError: inconsistent use of tabs and spaces in indentation

Does anyone know why the files are not written to the image and is it best to inherit devicetree into the actual kernel bbappend or have the recipe separate?