Fix debian installer

After this MR, the only thing blocking debian-installer from building is that trisquel-keyring-udeb is missing from our debian-installer/main repo.

I used a local repo to provide trisquel-keyring-udeb and was able to build mini.iso. I then tried using mini.iso to install Trisquel. The install failed because debootstrap-udeb is missing from our debian-installer/main repo.

I then compared etiona's and bionic's packages in debian-installer/main to see what other udeb packages we might need to build.

@ruben Please definitely build

• trisquel-keyring-udeb
• debootstrap-udeb

and then almost certainly build

• debian-installer-udebs
• openssh-client-udeb
• openssh-server-udeb

and then maybe build

• ubuntu-keyring-udeb