[Buildroot] [PATCHv2] Add some support for device tree kernels with appended trees

[Maxime Ripard <maxime.ripard@free-electrons.com>]

Hi everyone,

This is a refactoring of the previous basic device tree
options available for microblaze.

The previous option only made it possible for microblaze targets
to give a path to an external device tree, and build a simpleImage
with it. This involved using a custom target name as simpleImages
are built with the simpleImage.dt_name.
This is also the case on powerpc with cuImages.

This patchset replaces the existing mechanism with a more generic one.
First, it allows to build device tree sources to blobs, both taking
dts present in the kernel sources and external ones, using a path.

Then, it adds support for kernel images with appended device tree blobs.
This option is only for ARM because it is the only architecture I can
think of that has such feature.

Finally, it adds the simpleImage and cuImage as regular image variants,
with the necessary logic to append the device tree name to use, while
doing a bit of code factorisation for u-boot images along the way.

Thanks,
Maxime