We were using absolute imports under the assumption that the /scripts
directory is in the path. This worked in normal use because every one of our
Python scripts either were in the /scripts directory, or added the /scripts
directory to the module search path in order to reference mbedtls_dev.
However, this broke things like
```
python3 -m unittest scripts/mbedtls_dev/psa_storage.py
```
Fix this by using relative imports.
Relative imports are only supposed to be used inside a package (Python
doesn't complain, but Pylint does). So make /scripts/mbedtls_dev a proper
package by creating __init__.py.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
