Building aufs-util aufs4.9 currently does not build, because it does not find #include <linux/aufs_type.h>. This issue can be resolved by manually adding the include directory of build/aufs4.9 to aufs-util.mk. However, I don't know how to fix this issue correctly...
Seems to only happen if activating aufs after building kernel. Completely rebuilding the kernel resolved the issue for me