The last time I went looking, I found some stuff on NXP's site but downloading anything required registration/login to download. Taking a quick look back at the bookmark shows registration/login is still needed but it does mention 4.1.5 for the iMX6SLL.
IMXLINUX: Embedded Linux for i.MX Applications Processors
Don't know how much use this will be since I never downloaded anything but good luck!