Hello,
That depends on your end application. Yocto is a custom build embedded image, that means that you will build your Yocto image depending on the requirements that you need. For our development boards, Yocto should be the easiest, since Android is more difficult to modify and it does have several dependencies that a simple modification could make the image crash.
You can download our BSP for the i.MX6UL on our website. Also, you can download the required documentation to build the Yocto image.
i.MX 6UltraLite Applications Processor | Single Arm® Cortex®-A7 @ 696 MHz | NXP
i.MX 6UltraLite Applications Processor | Single Arm® Cortex®-A7 @ 696 MHz | NXP
Moreover, if you are interested in know more about how to customize your Yocto image, I highly recommend you to see the below document. There you could find step by step how to build your Yocto image.
https://community.nxp.com/docs/DOC-94849
Hope this information can help you.
Best regards,
Diego.