Enable dropbear on boot to decrypt full disk encryption

• install dropbear-initramfs
• put public rsa key in /etc/dropbear/initramfs/authorized_keys, prepend
  no-port-forwarding,no-agent-forwarding,no-x11-forwarding,command="/bin/cryptroot-unlock" ssh-rsa ...
• f DHCP no config neccesary
• in /etc/dropbear/initramfs/dropbear.conf
DROPBEAR_OPTIONS="-p 4789 -s -j -k -I 60"
• issue to add the keys and generate new initiramfs
  update-initramfs -u
• (old) writeup: 

https://hamy.io/post/0009/how-to-install-luks-encrypted-ubuntu-18.04.x-server-and-enable-remote-unlocking/