[yocto] Allow choose of U-Boot config for machine

[JH]

Hi,

The imx6ullevk.conf defined sdcard for booting on imx6 EVK:

imx6ullevk.conf

UBOOT_CONFIG ??= "sd"
UBOOT_CONFIG[sd] = "mx6ull_14x14_evk_config,sdcard"
UBOOT_CONFIG[mfgtool] = "mx6ull_14x14_evk_config"

I need to change it for a customized board referenced from imx6 EVK
but booting from NAND, there are some blogs said it can be done to add
UBOOT_CONFIG=nand either in local.conf or command line. I tried it did
not work:

ERROR: Unable to start bitbake server (None)
--- Starting bitbake server pid 2673 at 2019-07-02 11:54:24.011086 ---
ERROR: local.conf:34: unparsed line: 'UBOOT_CONFIG=nand'

What should be a proper process to overwrite imx6ullevk.conf
UBOOT_CONFIG from sdcard to nand?

Thank you.

Kind regards,

- JH