add device_kernel_headers module for vendor-specific kernel headers

device_kernel_headers is a built-in heder-only lib that provides device-specific
kernel headers. The header path is configured via a new product variable
SystemIncludeDirs, which is currently the mirror of
TARGET_PROJECT_SYSTEM_INCLUDES in the make world.

Note: generic kernel headers (bionic/libc/kernel) have been added to the
include path by default. "device_kernel_headers" module is for device-specific
kernel headers such as /device/*/*/kernel-headers.

Note 2: this is opt-in for Android.bp modules (i.e. header_libs :
["device_kernel_headers"] required.) while it is always provided to
Android.mk modules.

Bug: 62939405
Test: choosecombo to aosp_sailfish (or any other Pixel/Nexus targets)
BOARD_VNDK_VERSION=current m -j gralloc.msm8996 (or any other vendor
libs using vendor-specific kernel headers)

Change-Id: I81c60abc13942c89fff723d1544b27a81b300db0
diff --git a/cc/kernel_headers.go b/cc/kernel_headers.go
new file mode 100644
index 0000000..c66fca6
--- /dev/null
+++ b/cc/kernel_headers.go
@@ -0,0 +1,53 @@
+// Copyright 2017 Google Inc. All rights reserved.
+//
+// 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.
+
+package cc
+
+import (
+	"android/soong/android"
+)
+
+type kernelHeadersDecorator struct {
+	*libraryDecorator
+}
+
+func (stub *kernelHeadersDecorator) link(ctx ModuleContext, flags Flags, deps PathDeps, objs Objects) android.Path {
+	if ctx.Device() {
+		f := &stub.libraryDecorator.flagExporter
+		for _, dir := range ctx.DeviceConfig().DeviceKernelHeaderDirs() {
+			f.flags = append(f.flags, "-isystem"+dir)
+		}
+	}
+	return stub.libraryDecorator.linkStatic(ctx, flags, deps, objs)
+}
+
+func kernelHeadersFactory() android.Module {
+	module, library := NewLibrary(android.HostAndDeviceSupported)
+	library.HeaderOnly()
+
+	stub := &kernelHeadersDecorator{
+		libraryDecorator: library,
+	}
+
+	module.compiler = nil
+	module.linker = stub
+	module.installer = nil
+
+	android.InitAndroidArchModule(module, android.DeviceSupported, android.MultilibBoth)
+	return module
+}
+
+func init() {
+	android.RegisterModuleType("kernel_headers", kernelHeadersFactory)
+}