copy the vanilla kernel image to the target dir

I find it hard to use the vanilla kernel binary directly from
${VANILLA_LINUX_KERNEL}. Adding a target in the Makefile for the
vanilla kernel would make our client's scripts easier to maintain.

Change-Id: I1fa3931beb25a0d3199017d01388f195884ea4e4

oak_containers_kernel/Makefile

@@ -8,5 +8,10 @@ target/bzImage:
 	# Log digest of kernel in order to detect non reproducibility.
 	sha256sum target/bzImage
 
+target/vanilla_bzImage:
+	mkdir --parents target
+	cp $${VANILLA_LINUX_KERNEL}/bzImage target/vanilla_bzImage
+	sha256sum target/vanilla_bzImage
+
 clean:
 	rm --recursive --force target