Hello Ramakrishna Thumu,
To the list of useful resources Kunal mentioned, I would add the Linux for i.MX landing page:
https://www.nxp.com/design/software/embedded-software/linux-software-and-development-tools/embedded-...
I would recommend starting with the documentation for thee Kernel revision you are interested in. The latest Linux BSP release with support for the i.MX6SX would be L4.14.98_2.3.1. These BSP releases support NXP boards only. Once you download the documentation a good starting point is the included i.MX Yocto Project User’s Guide.
If you will be using a third-party board, it may also be good to check the Community BSP. You can see the documentation for this BSP on the link below:
http://freescale.github.io/
I hope this helps!
Regards,