Import Linux kernel to build driver module.

This is used to build the driver in continuous integration.

Change-Id: Idb12781505537b257cd9ec97b7c1f67a15dd74a2
diff --git a/.gitmodules b/.gitmodules
index e95a192..180293c 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -13,3 +13,7 @@
 	path = third_party/googletest
 	url = https://hafnium.googlesource.com/hafnium/third_party/googletest
 	shallow = true
+[submodule "third_party/linux"]
+	path = third_party/linux
+	url = https://hafnium.googlesource.com/hafnium/third_party/linux
+	shallow = true
diff --git a/driver/linux b/driver/linux
index 1ee3565..ce08e9e 160000
--- a/driver/linux
+++ b/driver/linux
@@ -1 +1 @@
-Subproject commit 1ee3565f71132930f3a67f5fc8edcb16cc0d6f46
+Subproject commit ce08e9ec0915563d60536c048708389e1345b5fe
diff --git a/kokoro/ubuntu/build.sh b/kokoro/ubuntu/build.sh
index faa0e22..c7951e4 100755
--- a/kokoro/ubuntu/build.sh
+++ b/kokoro/ubuntu/build.sh
@@ -43,9 +43,19 @@
 # Step 1: make sure it builds.
 #
 
-# Check the build works.
+# Check the hypervisor builds.
 make
 
+# Check the Linux kernel module builds.
+(
+export ARCH=arm64 &&
+export CROSS_COMPILE=aarch64-linux-gnu- &&
+make ${KOKORO_JOB_NAME+CC=${CROSS_COMPILE}gcc-4.8} \
+     -C third_party/linux defconfig modules_prepare &&
+cd driver/linux &&
+make ${KOKORO_JOB_NAME+CC=${CROSS_COMPILE}gcc-4.8}
+)
+
 #
 # Step 2: make sure it works.
 #
diff --git a/third_party/linux b/third_party/linux
new file mode 160000
index 0000000..bbc0e19
--- /dev/null
+++ b/third_party/linux
@@ -0,0 +1 @@
+Subproject commit bbc0e19aa259566f22c31e4dc344f88a1c759104