Cheating: For now first step is to get Arch Linux ARM running on Cubieboard. SSH into Arch install, update u-boot.
Download stage 3 ARMv5tel https://www.gentoo.org/downloads/#arm
sudo fdisk /dev/sdX
- +-150M, boot partition, needed for kernels etc.; type: b W95 FAT32
sudo mkfs.vfat -F 32 /dev/sdX1 -n BOOT
- rest; type: 83 Linux
sudo mkfs.btrfs -f -L Gentoo /dev/sdX2 -n 4096
sudo mkdir /mnt/gentoo
sudo mount -o async /dev/sdX2 /mnt/gentoo
async may improve or degrade performance
sudo tar -C /mnt/gentoo -xf stage3-armv5tel-*.tar.bz2
sudo mount /dev/sdX1 /mnt/gentoo/boot