To compile u-boot from git, you need the following packages installed on Debian or one of its derivatives: gcc-aarch64-linux-gnu build-essential bison flex device-tree-compiler
And then you compile it by running:
cp mntreform-config .config
env CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm make flash.bin
You can find precompiled u-boot as the artifact of the latest CI job here: Jobs · Reform / reform-boundary-uboot · GitLab
Or use a direct download link to the latest stable version (v3): https://source.mnt.re/reform/reform-boundary-uboot/-/jobs/artifacts/v3/raw/flash.bin?job=build