Thanks, I already know where the coreboot source is (and I'm already using it with Heads). Concerning Secure Boot, I only found this (emphasis mine):
> soc/amd/common/block/psp: Add platform secure boot support
>
> Add Platform Secure Boot (PSB) enablement via the PSP if it is not already enabled. Upon receiving psb command, PSP will program PSB fuses as long as BIOS signing key token is valid. Refer to the AMD PSB user guide doc# 56654, Revision# 1.00. Unfortunately this document is only available with NDA customers