Allow running source file generators from a subdirectory

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

scripts/mbedtls_dev/build_tree.py

@@ -0,0 +1,38 @@
+"""Mbed TLS build tree information and manipulation.
+"""
+
+# Copyright The Mbed TLS Contributors
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+
+def looks_like_mbedtls_root(path: str) -> bool:
+    """Whether the given directory looks like the root of the Mbed TLS source tree."""
+    return all(os.path.isdir(os.path.join(path, subdir))
+               for subdir in ['include', 'library', 'programs', 'tests'])
+
+def chdir_to_root() -> None:
+    """Detect the root of the Mbed TLS source tree and change to it.
+
+    The current directory must be up to two levels deep inside an Mbed TLS
+    source tree.
+    """
+    for d in [os.path.curdir,
+              os.path.pardir,
+              os.path.join(os.path.pardir, os.path.pardir)]:
+        if looks_like_mbedtls_root(d):
+            os.chdir(d)
+            return
+    raise Exception('Mbed TLS source tree not found')