cmake: modules: add generated_file_directories

Due to Hyrum's Law, there are users out in the wild depending on this
directory existing to place their own generated files, so it'd break
things unnecessarily to not do this if we don't have a DTS, as
explained in a source code comment.

However, this doesn't really have anything to do with DTS processing,
so split it into its own module to separate concerns. This isn't
really a CMake module in the usual sense of something that defines
functions you can use, and is therefore a form of technical debt. The
decision was made to accept this because fixing this is a larger task
for the files in cmake/modules/, since there are multiple other
examples of this in here.

This also paves the way for inserting another module in between the
generated_file_directories and dts modules that itself depends on
these directories existing.

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