Recipes to modify variable UBOOT_CONFIG?

Hi!

In our machines we've defined `UBOOT_CONFIG` for `sd` and `emmc` and set `UBOOT_CONFIG` to `emmc`. This works fine.

We got one image recipe where we need to run from SD-Card instead of eMMC, so we set set `UBOOT_CONFIG` to `sd` in that image recipe. When I run `bitbake -e <image>` I can see that `UBOOT_CONFIG` is set to `sd`. But in our custom appended task `do_unpack` of the uboot recipe it seems that `UBOOT_CONFIG` still remains to `emmc`.

When I grep through *all* recipes I couldn't find a single recipe that modifies `UBOOT_CONFIG`. So the question is whether it is possible to modify `UBOOT_CONFIG` outside the machines?

Thanks in advance,
H. Wulff