Hello Fernando A. E.,
There are a couple of BSP Releases for NXP’s boards based on kernel 4.X. You can see them on the following link:
There is also a Community BSP which is part of the Yocto Project and maintained by the community and this supports also 3rd party boards and kernel revisions different than the BSP Releases which are somewhat limited.
I would recommend looking for these kernels when working on the i.MX6UL as porting a kernel from mainline may be time consuming.
Regards,