cmake: Re-direct KCONFIG_ROOT when convention is used

Automatically detect the Kconfig file in it's conventional placement
${APPLICATION_SOURCE_DIR}/Kconfig and set KCONFIG_ROOT accordingly.

'Convention over configuration' is an imporant principle for
minimizing metadata and keeping things consistent. We use this
principle for instance with the file prj.conf.

This commit applies this principle to also re-direrct KCONFIG_ROOT.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>