On Ubuntu 16.04, your `rsyslog` rules would be found in the `/etc/rsyslog.d/50-default.conf` file. Inside that file, you'll see the two following lines:
*.*;auth,authpriv.none -/var/log/syslog
kern.* -/var/log/kern.log
You could skip the kernel messages from being logged to `/var/log/syslog` like so:
*.*;auth,authpriv,kern.none -/var/log/syslog
kern.* -/var/log/kern.log