Hi David!
Yes, if you're using LinuxBoot then you need to include all the drivers you wish to use in your LinuxBoot kernel, including SATA and USB.
`localboot -grub` will systematically mount partitions looking for a GRUB config file, parse it, and kexec the kernel it finds. You can select a particular entry using the `-config` command-line parameter.
Finally I've been able to boot my system and implement a signature verification using TPM2. It's very easy to try new things with linuxboot and u-root!
Thanks! Jorge