Hello Ken Green,
The latest Yocto BSP would be L4.14.98_2.2.0. The BSPs are delivered through Yocto, if you wish to build and customize your own image you would need to download the document bundle for this BSP.
The latest would be L4.14.98_2.2.0_LINUX_DOCS, available on the link below (you may need to login to download this file):
https://www.nxp.com/design/software/embedded-software/linux-software-and-development-tools/embedded-...
Inside the documentation you will find the i.MX Yocto Project User's Guide which shows how to setup your host to build the images.
I hope this information helps!
Regards,