Hey.
That is very much all about openwrt and the wiki provides all the information. When you build the image just ad your configuration files to "backfire/package/base-files/files/etc/config/
Don't do that. Not all targets use the base-files configs and its a nuissance to have local changes while trying to stay in sync with the repo.
The proper wqay to overlay the build with custom configs is to create a toplevel files/ directory and put the configs there, e.g.
files/etc/rc.local files/etc/config/network
and so on. Toplevel here means the directory where you run make in.
Regards, Jow