imx6: kernel update for nitrogen6x

Correctly append new kernel configs for the updated kernel version
and include device tree files in the boot image.

Change-Id: I3d7c826e3e96338048e82b23a6723212e4adbd03
Reviewed-by: Kalle Viironen <kalle.viironen@digia.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>