CI: Script to convert CI variables

Add a script to convert the contents of ci/vars.yml to Azure Pipelines
logging commands, which effectively set runtime variables in a pipeline.

We need this script as a workaround for a missing Azure Pipelines
feature: variables are not inherited in extended templates, and reading
the vars.yml file in a "extends" template isn't possible either (at
least not if we want to use the exact revision of the ibex repository
which triggered CI).

ci/vars_to_logging_cmd.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+# Copyright lowRISC contributors.
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
+# Read an Azure Pipelines-compatible variables file, and convert it into
+# logging commands that Azure Pipelines understands, effectively setting the
+# variables at runtime.
+#
+# This script can be used as a workaround if variables cannot be included in the
+# Pipeline definition directly.
+#
+# See https://docs.microsoft.com/en-us/azure/devops/pipelines/scripts/logging-commands
+# for more information on logging commands.
+
+import sys
+import yaml
+
+def vars_to_logging_cmd(vars_file):
+    data = {}
+    print(vars_file)
+    with open(vars_file, 'r', encoding="utf-8") as fp:
+        data = yaml.load(fp, Loader=yaml.SafeLoader)
+
+    if not (isinstance(data, dict) and 'variables' in data):
+        print("YAML file wasn't a dictionary with a 'variables' key. Got: {}"
+            .format(data))
+
+    print("Setting variables from {}".format(vars_file))
+    for key, value in data['variables'].items():
+        # Note: These lines won't show up in the Azure Pipelines output unless
+        # "System Diagnostics" are enabled (go to the Azure Pipelines web UI,
+        # click on "Run pipeline" to manually run a pipeline, and check "Enable
+        # system diagnostics".)
+        print("##vso[task.setvariable variable={}]{}".format(key, value))
+
+    return 0
+
+if __name__ == "__main__":
+    if len(sys.argv) < 2:
+        print("Usage: {} VARS_FILE".format(sys.argv[0]))
+        sys.exit(1)
+    sys.exit(vars_to_logging_cmd(sys.argv[1]))