For running test on imx6 EVK, it was booting from sdcard as a default UBOOT_CONFIG, now I need to build Yocto image for a customized board based on imx6ull EVK but booting from NAND, the UBOOT_CONFIG should be changed to nand, what is the appropriate configuration to overwrite imx6ullevk UBOOT_CONFIG?
UBOOT_CONFIG ??= "sd"
UBOOT_CONFIG[sd] = "mx6ull_14x14_evk_config,sdcard"
UBOOT_CONFIG[mfgtool] = "mx6ull_14x14_evk_config"