Dunfell u-boot build issue (git is not clean)

Hi all;

I'm attempting to upgrade a build system that has Thud era recipes to one
that targets Dunfell. We have a custom kernel and a custom u-boot (based on
2020.4) so we have our own recipes for building those (although they
piggyback on the Freescale/OpenEmbedded base.)

What I've run into is that when building u-boot inside of Yocto, it
complains that "git is not clean, please run 'make mrproper'." As I can
cleanly build u-boot outside of Yocto, I assume there's something wrong
with my recipes.

I found https://github.com/Freescale/meta-freescale/issues/280 which points
to
http://cgit.openembedded.org/openembedded-core/commit/meta/recipes-bsp/u-boot?h=master-next&id=26023b6b0f897842fd98b3e10a8acd5b3ad8f418
but the issue is resolved without any comment on what was done to fix it,
and I haven't found any further reference to that oe commit.

If this sounds familiar to anyone, I'd love some breadcrumbs!

Thanks,
Matt Walker