Add in-kernel test runner

This adds a way to run tests inside the Smalltown kernel.

Improvements to the Bazel part of this are tracked in T726

Test Plan: Tested by intentionally failing the test.

X-Origin-Diff: phab/D485
GitOrigin-RevId: e4aad7f28d122d82a7fcb6699e678cbe022e2f73
diff --git a/core/tools/kconfig-patcher/kconfig-patcher.bzl b/core/tools/kconfig-patcher/kconfig-patcher.bzl
new file mode 100644
index 0000000..a6af343
--- /dev/null
+++ b/core/tools/kconfig-patcher/kconfig-patcher.bzl
@@ -0,0 +1,33 @@
+#  Copyright 2020 The Monogon Project Authors.
+#
+#  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.
+
+"""Override configs in a Linux kernel Kconfig
+"""
+
+def kconfig_patch(name, src, out, override_configs, **kwargs):
+    native.genrule(
+        name = name,
+        srcs = [src],
+        outs = [out],
+        tools = [
+            "//core/tools/kconfig-patcher",
+        ],
+        cmd = """
+        $(location //core/tools/kconfig-patcher) \
+            -in $< -out $@ '%s'
+        """ % struct(overrides = override_configs).to_json(),
+        **kwargs
+    )