[TF-M] CMake flags

Hi Raef. Thanks for the presentation today.

If I remember correctly, TF-M's CMake has quite a lot of command line flags now (the ones that are set with -D). And I think this will grow in the future as more options are needed.

From a command line point of view, this is kind of annoying because the commands end up being quite long. Do you guys think something like Kconfig could help?

Adrian