ml-apps: Fix building different ML apps subsequently

When building ML apps, the MLEK downloads model resources
for the model specified by a given application.
The FRI integration of the MLEK prevents these resources
from being re-donwloaded if they already exist to speed up
build time.

However, in the case of a different application being build,
model resources have to be re-downloaded as different ML
applications use different models.
To allow the re-download of model resources, the generated
ML model resources subdirectory is now generated in the CMake
build directory.

Building a different application requires triggering
a clean build which will also delete the resources specific
to a model used by another application.

Signed-off-by: Hugues Kamba Mpiana <hugues.kambampiana@arm.com>