if you install 2.6 with ROOTFS=xfs exported to the environment and the xfsprogs added, it won't boot because the xfs module is missing its dependencies.
The files "libcrc32c.ko" and "crc32c.ko" need to be added to the initfs image. I wasn't able to figure out how to add "crc32c.ko". I added both to /etc/mkinitfs/modules.d/xfs but "crc32c.ko" isn't added at all and not part of the initfs image. When doing a dry run, listing all files that would be included, it is listed. But when the system tries to boot later, it errors out and loading "crc32c.ko" manually gives me an "modprobe: module crc32c.ko not found in modules.dep".
Here a screenshot showing the main error:
And here one with the errors when trying to load "xfs.ko":