1. Edit the /etc/rsyslogd.conf file for allowing the rsyslog server to accept logs from UDP (port 514), by uncommenting the following lines:
# provides UDP syslog reception $ModLoad imudp $UDPServerRun 514
By default, rsyslogd will treat the remote logs as if they were local ones (meaning, among other things, the entries will end up in the same logfiles where the local logs are stored). In my setup, I found convenient to override that behaviour and instead place all the logs from my router in a different file:
2. (Optional) Create the file /etc/rsyslogd.d/openwrt.conf with the following contents:
:source, isequal, "myrouter.mydomain" /var/log/mylogfile.log :source, isequal, "myrouter.mydomain" ~
The first line logs the entries coming from „myrouter.mydomain” (can be expressed as an IP as well) into /varlog/mylogfile.log, while the second one prevents any further processing of the entry by discarding it (note the final tilde ~). A more complete description of the different configuration options  and example configurations  can be found on the rsyslogd man pages and wiki.
Hope it helps someone.