doc: Process zephyr modules to parse all of Kconfig

In order to parse all the Kconfig files that make up the whole Kconfig
tree we need to process the modules, since this can optionally provide
Kconfig files. In order to do so, include the modules CMake file so that
they are listed and a Kconfig.modules is generated.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>