Add BUILD.gn

Move build rule for the Linux kernel from //third_party to its own
folder. This way the kernel will be built in its own folder rather than
together with other third_party/* projects, avoiding possible clashes.

Change-Id: I90f2cb03abe90ca091970fc9f69ce61e79cd6d82
diff --git a/BUILD.gn b/BUILD.gn
new file mode 100644
index 0000000..5651f72
--- /dev/null
+++ b/BUILD.gn
@@ -0,0 +1,19 @@
+# Copyright 2019 The Hafnium Authors.
+#
+# 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
+#
+#     https://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("//build/linux/linux.gni")
+
+linux_kernel("linux") {
+  prebuilt = "//prebuilts/linux-aarch64/linux/vmlinuz"
+}