Hello,
With BSP you use Yocto Project with repo utility to fetch all the sources (layers) and then, in the process of building an image, you fetch the source packages, build them, etc.
But NXP source code is in http://git.freescale.com/git/
Follow the instructions in Freescale_Yocto_Project_User's_Guide.pdf document to build an image.
Luis
