linux: Fix uImage with appended DTs generation

Fixes bug #5516 - appended device tree blobs on uImage fails

Before version 3.7 of the kernel, building the zImage and then the
uImage will rewrite the zImage in the process, removing the device tree
we just appended.

Use mkimage to append the device tree to the uImage and rebuild the
headers directly.

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