you want an image with a custom /etc/config/firewall or a custom etc/sysctl.conf, then create this files as:
<buildroot>/files/etc/config/firewall
<buildroot>/files/etc/sysctl.conf
E.g. if your <buildroot> is ~/source and you want some files to be copied into firmware image's /etc/config directory, the correct place to put them is ~/source/files/etc/config.