Don't try to resolve SSH DNS source

Often makes ssh connections take ages.
Add mDNS (avahi)

This allows you to connect to the device with:

  ssh linaro@linaro-developer.local.

Closes #7.
Set the default hostname to $BOARD
Replace "linaro" user with "mirage" one

Also, pre-initialise OPAM.
Explain about mDNS name in README
Added missing build dep on dosfstools
Make the image smaller by expanding the ext4 partition on first boot
Don't start services while building image
Regenerate SSH keys on first boot
Merge pull request #10 from talex5/avahi

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