Servald configuration files can be found at both /etc/serval/serval.conf and /serval-var/serval.conf. The latter file is loaded by Servald when it starts. Servald needs to be restarted with servald stop && servald start for changes to take effect.
If /serval-var/serval.conf is not present at boot, /etc/serval/serval.conf is copied over by /etc/rc.d/S49servald.
See the current default configuration file for Mesh Extenders and the Servald documentation for details on the contents of the file.