Hello Anil Kumar,
You can build customized images for the i.MX6SX SABRE Board using the Linux BSP Release (link below, please note that you may need to login in order to download this file).
https://www.nxp.com/webapp/Download?colCode=L4.1.15_2.0.0-LINUX-DOCS
Please follow the Yocto User’s Guide document contained in this bundle. Since Yocto is used as a means of distribution the source code is fetched on demand. You may, however, fetch the required sources for a given image by using the following command:
bitbake <image_or_recipe> -c fetchall
You may also extract the toolchain using:
bitbake meta-toolchain
Once fetched you can find the source code on the work directory:
/<BSP_DIR>/<BUILD_DIR>/tmp/work/
For example, the source for the kernel would be on:
/<BSP_DIR>/<BUILD_DIR>/tmp/work/<MACHINE>/linux-imx/<KERNEL_VER>/git/
The sources are located depending on the recipes that compiles and install the packages so it may not be very straightforward.
I hope this information helps!
Regards