mini-os: build fixes

mini-os: build fixes

Make .o rules depend on the includes. Before, only the final link step
depended on setting up the includes directory, making parallel builds
unreliable.

Make symlinks use explicit make rules instead of using a phony target.
Avoids unnecessary rebuilds.

[talex5@gmail.com: bring back "make links", for stubdom]
Signed-off-by: Thomas Leonard <talex5@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>