Use relative imports when importing other modules in the same directory

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>

scripts/mbedtls_dev/__init__.py

@@ -0,0 +1,3 @@
+# This file needs to exist to make mbedtls_dev a package.
+# Among other things, this allows modules in this directory to make
+# relative impotrs.