Hello Martin Dusek,
You would be using linux-imx as your kernel, from what I understand. The recipes on the BSP Release or Community BSP already use this as the kernel for all the NXP Machines.
You would need to change the linux-imx_<VERSION>.bb information on where to obtain the source code. For example for 4.1.15’s BSP would be from linux-imx_4.1.15.bb, which is located on the following path of the BSP Release:
<FSL-RELEASE-BSP >/sources/meta-fsl-bsp-release/imx/meta-bsp/recipes-kernel/linux/ linux-imx_4.1.15.bb
As for the device tree you should be able to add the instructions on adding it on the same recipe as it already includes linux-dtb.inc, where the default dtb files are included.
I hope this general guidelines help!
Regards,