kconfig: add support for warnings when enabling deprecated features

This adds two new Kconfig settings.

The first setting `DEPRECATED` which is a promptless symbol.
This symbol must be selected by any deprecated setting when enabled.

The second setting is `WARN_DEPRECATED` which is a user controlled
setting that configures the build system to print a warning when a
deprecated feature is enabled.

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