cmake: Add variable for the TFM build directory

The TFM build directory path is hardcoded in many places.
In order to support out-of-tree secure partitions the output path
has to be known in potentially out-of-tree build scripts.
This could potentially break out-of-tree build scripts if the
location of the build directory was changed.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>