No longer execute hostname command if hostname option is not present

No longer execute hostname command if hostname option is not present

There's no need to execute the hostname command if the boot
option hostname is not present as /etc/init.d/hostname.sh is
executed on Grml during bootup. We keep the config_hostname()
only because we support setting it to a random hostname if no
argument was specified to the boot option and to keep using
grml-hostname as main interface for changing hostnames
on-the-fly.