kbuild: Add make dir-pkg build option

Add a 'dir-pkg' target which just creates the same directory structures
as in tar-pkg, but doesn't package anything.
Useful when the user wants to copy the kernel tree on a machine using
ssh, rsync or whatever.

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>