kconfig: soc and shield cleanup

Always source the Zephyr base soc and shield (board root) Kconfigs
directly from Kconfig instead of generated Kconfig files.

This has the benefit that it is no longer necessary to generate Kconfig
files to source SoC root and shield (board root) when no custom roots
are provided.

Also this cleans up the doc/CMakeLists.txt and ensures that the
doc/CMakeLists.txt is not getting out of sync with the Kconfig.cmake.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>