Using the latest (1.5) mtd-utils from git://git.infradead.org/mtd-utils.git I have tried to use the sdk (3.00.03.15) images to simply recreate the base ubi filesystem.
After decompressing the cpio.gz file, I run
mkfs.ubifs -r ./filesystem/ -o ubifs.img -F -m 2048 -e 126976 -c 936
followed by: ubinize -o keystone-evm.ubi -m 2048 -p 128KiB -s 2048 -O 2048 ubinize.cfg
Where the ubinize.cfg is the same as what came with the sdk, except renamed the rootfs file to ubifs.img.
I have attached the boot log where the kernel has trouble mounting the rootfs. I have verified that if I reflash the prebuilt keystone-evm-ubifs.ubi the kernel boots just fine.