Allow framework scripts to import modules from the main project

Allow framework scripts to import scripts from the main project as modules,
for example config.py.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/project_scripts.py

@@ -0,0 +1,17 @@
+"""Add the consuming repository's scripts to the module search path.
+
+Usage:
+
+    import project_scripts # pylint: disable=unused-import
+"""
+
+# Copyright The Mbed TLS Contributors
+# SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+#
+
+import os
+import sys
+
+sys.path.append(os.path.join(os.path.dirname(__file__),
+                             os.path.pardir, os.path.pardir,
+                             'scripts'))