To start icecast at system boot add:

icecast_enable="YES"

to /etc/rc.conf. See the icecast -h command for optional additional
flags. To specify a config file for example add:

icecast_flags="-c /usr/local/etc/icecast.xml"

Customize /usr/local/etc/icecast.xml for your environment.  In
particular if you use chroot in the config file ensure the pid
directive in icecast.xml matches the setting in
/usr/local/etc/rc.d/icecast.sh.