scripts: add new west build "build.cmake-args" config option

This option, if set, will add arguments to CMake whenever a new build
system is being generated.

It doesn't affect other invocations of CMake, such as when cmake(1) is
run in build tool mode to actually compile the application.

See the documentation changes for details.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>