@purpleidea the system boot section here a bit further down needs to be updated because there are more options and esp. cpu modules now, and integration with debian's initramfs, but it will give you a general impression https://mntre.com/reform2/handbook/advanced.html
and of course https://docs.u-boot.org/en/latest/
our OS build process is here https://source.mnt.re/reform/reform-system-image