flash raspian image from Libre Computer to both microSD cards Download

insert one of the microSD cards you flashed into the Le Potato and boot (follow initial setup as required)

insert the second microSD into usb adapter and insert adapter into usb slot on Le Potato, this is where you will make changes and install Gentoo

delete root partion (second partition)

create a swap partition if desired

create a new root partition

mkfs.btrfs on the new root partition

mount new partition at /mnt/gentoo (mkdir if needed)

mount the existing EFI partition on /mnt/gentoo/boot/efi (mkdir first)

cp -rp the firmware, kernel modules, kernel headers and the rest of /boot from the raspian microSD you are booted on to the one on the adapter to the same path

use lsmod and put list of modules into /etc/modules-load.d/raspian-modules.conf

at this point you can mostly follow the normal gentoo amd64 handbook for setup (use end0 for ethernet device)

make sure you adjust /etc/inittab if you want to use the uart interface

# Architecture specific features
f0:12345:respawn:/sbin/agetty 115200 ttyAML0 vt100

when you get to bootloader installation/configuration emerge grub with efi-64 (add following to /etc/portage/make.conf)

GRUB_PLATFORMS="efi-64"

grub-mkconfig -o /boot/grub/grub.cfg

grub-mkstandalone -O arm64-efi -o /boot/efi/EFI/BOOT/BOOTAA64.EFI “boot/grub/grub.cfg=/boot/grub/grub.cfg”