As distributed with FreeBSD u-boot does not save environment variables
when they need to be changed

This article offers instructions for re-compiling u-boot so it will
save the changes to uboot.env:
https://forums.freebsd.org/threads/rpi3b-u-boot-saveenv-fails.84728/

After applying the changes suggested and recompiling u-boot-rpi2 with
Poudriere the completed binary was installed in /boot/efi, but the Pi
didn't boot, merely hanging silently after power-cycling the hard disk.

Another link that might be relevant:
https://raspberrypi.stackexchange.com/questions/119822/how-to-save-and-compare-u-boot-environment-and-fw-env-config
and
https://stackoverflow.com/questions/70286457/unknown-command-saveenv-error-in-u-boot