Add support for appended device tree blobs for arm

This patch adds support for the ARM-only appended device tree
mechanism present in the kernel.

This option allows to add at the end of the kernel image the
device tree blob so that we can still boot device tree enabled
kernels with old bootloaders.

This patch also adds the needed logic to genereate such an image
when building zImages or uImages, also adding the necessary parts
to rebuild the uImage.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>