You can configure any port for YaCy but small numbers below 1024 are reserved to be used by root on linux. Because some firewalls block port 8090 it can be useful to redirect the server port 80 to 8090.
One solution for this is the usage of a port forwarding using iptables. As user root just do
iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8090
This is a temporary port redirection, if you want this to be permanent just write this line into /etc/rc.local