Add Avahi, set hostname, mirage user, smaller image, fresh SSH keys

To connect now use the board name, e.g.

    ssh mirage@cubieboard2.local.

The image is now further reduced to 1GB (got bored waiting for it to write each time). It expands / to 3GB on first boot and then uses the rest for LVM.

It also now regenerates the SSH keys, otherwise they'd always be the same in the pre-built binaries.

I also added `UseDNS no` to the ssh configuration, as that usually speeds things up.