Kconfig.modules needs to be at the top level build folder

Today we are generating `Kconfig.modules` inside `build/zephyr` but that is not accurate to what it represents. Since it is a result of running `west list` on the forest, this should not be tied to zephyr and should instead be at the top-level `build/` folder.
Now that we have an actual module in the upstream tree (tinycbor) it only makes sense that we have the list of modules where it belongs.