Change default setting and configuration during building image for 15.0

[hnyman]

@JerryBlue

I want to make settings configured during the image built so that the settings stay on router reset.

You can include them as custom files into the image.

Build system usage

( Or if you use just the imagebuilder instead of the full toolchain, you can still do the same: https://openwrt.org/docs/guide-user/additional-software/imagebuilder#files_variable