It seems that imx6ul architecture is not supported in 4.1.15 nxp kernel even though it says it does.
Either that or the support is incomplete and patches for the imx6ul cpu, freq etc are needed.
I could not boot my imx6ul with the 4.1.15-2.0.0_ga kernel. It halts at "Starting kernel in secure mode" and from my past experiences it means most likely that imx6ul cpu specific configuration is missing in the kernel.
Could you please confirm this or not?