dd if=/dev/zero bs=1M count=5120 of=rootfs_5GB.img
/sbin/mkfs.ext3 rootfs_5GB.img 
mount -o loop rootfs_5GB.img /mnt
sudo apt-get install debootstrap
debootstrap --arch=arm64 trusty /mnt
chroot /mnt
(chroot) # passwd
(chroot) # exit
umount /mnt

things to consider:

• Disable ureadahead.conf in init
• Add hvc0.conf to init
• Enable ssh