Hi Guix!

Here's a patch to update u-boot to 2021.07. I've tested this by booting
a barebone image on the pinebook pro. I've also tested it on the
rockpro64 however that still requires another fix, see bug#49550 [0]
which is an existing issue.

Thanks,
Pierre

[0]: https://lists.gnu.org/archive/html/bug-guix/2021-07/msg00225.html