References:
1) Prepare the UBIFS partiotion
ubiformat /dev/mtd7 -s 2048 -O 2048 ubiattach /dev/ubi_ctrl -m 7 -O 2048 ubimkvol /dev/ubi0 -s 235MiB -N rootfs mount -t ubifs ubi0:rootfs /mnt
2) install a root file system to /mnt directory and sync disks.
U-boot does not need to change any environment variables to boot from NAND