Build image with custom config files

Hi guys, I have a TP LINK WR940N V4 (4 MB FLASH), I already have many images compiled by myself with different packages depending of my requirements at the moment, but I urgently need a way to make a image with my own config files already stablished as default, like /etc/config/wireless or /etc/config/system, because I really don't know how to make a backup or upgrade version via CLI (I know to do that with LUCI, but that package consumed me a lot of flash memory [I always upgrade via TFTP recovery option]), I am very bad using the command line interface and I lose a lot of time configurating all my devices any time I need to do a change.

Put your desired custom config(s) in custom files directory