[yocto] Move device tree generation from include file to bbclass

[Bach, Pascal]

Hi

As ARM now also moved to device tree it look like in future we will have more kernels that are using device tree then ones that are not.
As far as I understand currently the generation of device trees is controlled via KERNEL_DEVICETREE and is handled in via an include file recipes-kernel/linux/linux-dtb.inc.

I was thinking about moving this include into a class so it becomes easier to use. Before I dive into implementing something I would like some feedback from the community.

I have the following variant in mind.

Add the device tree generation to the current kernel.bbclass (or let kernel.bblcass inherit from a kernel-dtb.bbclass).
This way all kernels would automatically be DT enabled.
The class would check if KERNEL_DEVICETREE is set and generate device trees based on this information. For boards that don't have KERNEL_DEVICETREE set the class would do nothing and the behavior is like before.
The advantage I see with this approach is that the only thing a user needs to do is to set KERNEL_DEVICETREE in the board and make sure the device trees are available in the kernel they like to build.

I appreciate your feedback?

Regards
Pascal