fix(cros-kernel2.eclass): Add kernel modules to initramfs.

Since we need to both bundle modules into the initramfs as well as
bundle the initramfs into the kernel image we need to update a pre-built
image with the user space tools as part of the kernel build process.

This seemed the best scheme, the alternatives were:
 - Unpack bootengine.cpio to a temporary directory, build and install
   kernel modules into that temporary directory, pass that directory
   plus a config file listing what device nodes to the kernel build.
 - Build kernel modules and generate a fresh bootengine.cpio using the
   update-bootengine tool. This would require calling sudo (and breaking
   out of the sandbox in the process) in the middle of the ebuild.

sdk_container/src/third_party/coreos-overlay/eclass/cros-kernel2.eclass

@@ -91,6 +91,26 @@ install_kernel_sources() {
 		"${D}/${dest_build_dir}/Makefile" || die
 }
 
+# @FUNCTION: update_bootengine_cpio
+# @DESCRIPTION:
+# Append files in the given directory to the bootengine cpio.
+# Allows us to stick kernel modules into the initramfs built into bzImage.
+update_bootengine_cpio() {
+	local extra_root="$1"
+	local cpio_path="$(cros-workon_get_build_dir)/bootengine.cpio"
+	local cpio_args=(--create --append --null
+		# dracut uses the 'newc' cpio format
+		--format=newc
+		# squash file ownership to root for new files.
+		--owner=root:root
+	)
+
+	echo "Updating bootengine.cpio"
+	(cd "${extra_root}" && \
+		find -depth -print0 | cpio "${cpio_args[@]}" -F "${cpio_path}" || \
+		die "cpio update failed!")
+}
+
 kmake() {
 	local kernel_arch=$(tc-arch-kernel)
 
@@ -127,6 +147,17 @@ cros-kernel2_src_configure() {
 }
 
 cros-kernel2_src_compile() {
+	# Build both vmlinux and modules (moddep checks symbols in vmlinux)
+	kmake vmlinux modules
+
+	# Install modules and add them to the initramfs image
+	local bootengine_root="${T}/bootengine"
+	kmake INSTALL_MOD_PATH="${bootengine_root}" \
+		  INSTALL_MOD_STRIP="--strip-unneeded" \
+		  modules_install
+	update_bootengine_cpio "${bootengine_root}"
+
+	# Build the final kernel image (bzImage)
 	kmake
 }