I made the following changes and tried clean everything to rebuild.
UBOOT_CONFIG = "nand" in "conf/local.conf" file
UBOOT_CONFIG[nand] = "mx6ul_14x14_evk_nand_config,ubifs" in "sources/meta-freescale/conf/machine/imx6ulevk.conf" file
My build command:
DISTRO=fsl-imx-fb MACHINE=imx6ulevk source imx-setup-release.sh -b build
bitbake core-image-base
But no NAND u-boot can be built anyway.