Undefining a variable in a recipe?

[Alex J Lennon <ajlennon@dynamicdevices.co.uk>]

Hi,

I have an issue with the build of an old u-boot (2009.08) which is
failing a check in poky as both UBOOT_CONFIG and UBOOT_MACHINE are being
defined for some reason.

poky/meta/classes/uboot-config.bbclass

 ubootmachine = d.getVar("UBOOT_MACHINE", True)
 ubootconfigflags = d.getVarFlags('UBOOT_CONFIG')

..

 if ubootmachine and ubootconfigflags:
       raise bb.parse.SkipPackage("You cannot use UBOOT_MACHINE and
UBOOT_CONFIG at the same time.")


I have a .bbappend on the original u-boot recipe and could solve the
problem by undefining UBOOT_MACHINE if I could work out how to do this
in the .bbappend

I've tried setting it to None or an empty string, and I tried an
anonymous python function but those approaches didn't help,

e.g. (in the .bbappend

UBOOT_MACHINE = ""

or

UBOOT_MACHINE = None

or

python __anonymous () {
  bb.data.delVar('UBOOT_MACHINE_imx6qsabresd')
}

I could just copy the original recipe from metal-fsl-arm into my own
layer and change it there I guess but I'd like to understand how to
achieve this with a .bbappend if it is possible as it seems cleaner.

Many thanks,

Alex