coreos-kernel.eclass: Adapt install_build_source for compatibility

build/source, which is accidentally an empty directory, needs to be a
symlink so that Gentoo kmod ebuilds can build when setting
KERNEL_DIR=/lib/modules/.../build. They detect the proper layout with
the symlink.

The other issue is building the Nvidia driver with
SYSSRC=/lib/modules/.../build. This works on Ubuntu but fails on
Flatcar. Ubuntus build directory contains symlinks to includes from the
source tree so recreate the same layout.

Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

sdk_container/src/third_party/coreos-overlay/eclass/coreos-kernel.eclass

@@ -243,6 +243,22 @@ install_build_source() {
 		--owner=root:root \
 		--dereference \
 		"${D}/usr/lib/modules/${KV_FULL}" || die
+	# ./build/source is a symbolic link so cpio ends up creating an empty dir.
+	# Restore the symlink.
+	pushd "${D}/usr/lib/modules/${KV_FULL}"
+	rmdir build/source || die
+	ln -sr source build || die
+	# Symlink includes into the build directory to resemble Ubuntu's /lib/modules
+	# layout. This lets the Nvidia driver build when passing SYSSRC=/lib/modules/../build
+	# instead of requiring SYSOUT/SYSSRC.
+	{
+		find source/include -mindepth 1 -maxdepth 1 -type d
+		find source/arch/${kernel_arch}/include -mindepth 1 -maxdepth 1 -type d
+	} | while read src; do
+		dst="${src/source/build}"
+		ln -sr "${src}" "${dst}" || die
+	done || die
+	popd
 }
 
 coreos-kernel_pkg_pretend() {