devicetree.bbclass: User/BSP device tree source compilation class

This bbclass implements the device tree compilation for user provided
device trees. In order to use this class, it should be inherited in a
BSP recipe which provides the sources. The default setup enables
inclusion of kernel device tree sources (though can be disabled by the
recipe by overriding DT_INCLUDE or KERNEL_INCLUDE).

This provides an additional mechanism for BSPs to provide device trees
and device tree overlays for their target machines. Whilst still
enabling access to the kernel device trees for base SoC includes and
headers.

This approach to providing device trees has benefits for certain use
cases over patching the device trees into the kernel source.

* device trees are separated from kernel source, allows for selection of
kernel and or kernel versions without needing to explicitly patch the
kernel (or appending to the kernel recipes).

* providing device trees from separate sources, from the layer,
generated by the recipe or other recipes.

This class also implements some additional features that are not
available in the kernel-devicetree flow. This includes population of
device tree blobs into the sysroot which allows for other recipes to
consume built dtbs (e.g. U-Boot with EXT_DTB compilation), device tree
overlay compilation and customizing DTC compilation args (boot
cpu/padding/etc.).

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Acked-by: Martin Hundebøll <mnhu@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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