Doc build does not include the zephyr modules Kconfig files

When building the doc, the `Kconfig.*` files in the modules (`Kconfig.tinycbor` at this point) are not being included due to the documentation build system currently creating an empty `Kconfig.modules` file:

`file(TOUCH ${CMAKE_CURRENT_BINARY_DIR}/Kconfig.modules)`

So we need to actually process the modules to populate the `Kconfig.modules` file.

