tfm: Refactor the TF-M Cmake code to simplify and remove the function

The current CMakeLists.txt contains a function that is called from
the same file.

This patch removes the abstraction, allowing to remove many
lines of parameter handling.

Additionally, with this patch, the Cmake argument handling is now
done via a list, which removes many more named variables.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>