scons: Add a build script for generating a root Kconfig file.

This root Kconfig file "source"s (includes) the base gem5 src/Kconfig
file, and also any optional Kconfig files found in the base of EXTRAS
directories. These will be called out in the menuconfig interface and
config files with the name of the EXTRAS directory they came from, and a
blank section will be present either if the Kconfig didn't exist, or it
did exist but had no options in it.

Change-Id: I54060d613f0e0ab9372bed37a2fe5849bf5bbcdb

build_tools/kconfig_base.py

@@ -0,0 +1,55 @@
+#! /usr/bin/env python3
+#
+# Copyright 2022 Google LLC
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met: redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer;
+# redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution;
+# neither the name of the copyright holders nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+import argparse
+
+from code_formatter import code_formatter
+
+parser = argparse.ArgumentParser()
+parser.add_argument("output", help="path of generated base Kconfig file")
+parser.add_argument("main", help="relative path to the main gem5 Kconfig file")
+parser.add_argument("extras_dirs", nargs="*", help="EXTRAS paths")
+
+args = parser.parse_args()
+
+code = code_formatter()
+
+code(
+    f"""# Automatically generated base Kconfig file, DO NOT EDIT!
+
+source "{args.main}"
+"""
+)
+
+for extras_dir in args.extras_dirs:
+    code(
+        f"""
+osource "{extras_dir}/Kconfig"
+"""
+    )
+
+code.write(args.output)