Disable root account

Currently on the devkit image, the root account is enabled and has a password set in /etc/shadow. We should disable this account by default and rely on sudo for root access. Otherwise the fact that we ship all units with the same default root password will become a security issue once that password is cracked (which should be easy given it's using standard crypt instead of SHA256 or SHA512).

To disable the account, /etc/shadow should be edited and the second field in the root account's line should be changed from the password hash to a !