Makefile: use order-only dependency so symlinks are made only once

Makefile: use order-only dependency so symlinks are made only once

The rule to create the staging symlink has it depend on BASE_DIR, and
the symlink is created in BASE_DIR, which means that when the symlink
is created, BASE_DIR is updated, and thus made more recent than the
symlink itself.

As a consequence, every time one runs 'make', the symlink will be older
than BASE_DIR, and so will be re-created.

Ditto for the host symlink when the user has elected to have an
out-of-tree host dir.

Fix that by changing to using an order-only dependency.

Signed-off-by: Danomi Manchego <[email protected]>
Signed-off-by: Yann E. MORIN <[email protected]>