When running `ego boot update` on my zen3 machine, I get the following warning:

  * WARN:   AMD system detected - please emerge sys-kernel/linux-firmware (with USE="initramfs") and run boot-update again; boot-update will then patch your system with the latest AMD
          CPU and chipset microcode patches at boot-time, protecting you against important vulnerabilities and errata.

Interestingly enough, if I mount /boot beforehand, things appear to work correctly and I get the following output:

  * NOTE:   AMD microcode will be loaded at boot-time. 

Peeking at /boot/grub/grub.cfg reveals that the warning isn't spurious and microcode isn't properly added to grub.cfg unless boot is mounted, which shouldn't be the case.