Rework support for the device tree

This patch introduces some support for device tree-enabled kernels.

It replaces the former BR2_LINUX_KERNEL_DTS_FILE option that was
microblaze-only, that was quite limited. This option was quite
limited, first obviously because it was restricted to microblaze,
but also because it targetted only external device tree source files,
and allowed only to build simpleImages using the custom image name
mechanism.

This patch adds a much more generic one, that can work on basically
every architecture that supports device tree. It allows to build
both device tree source file that comes with the kernel source or to
set the path to the device tree file to use so that one can use a
custom device tree.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>