kernel-devicetree: Fix interaction when packaging disabled

When packaging is disabled using the nopackages class, ensure we don't
add to PACKAGES. This fixes builds where we have an unpackaged kernel
alongside a packaged kernel.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d6b114cf)
Signed-off-by: Steve Sakoman <steve@sakoman.com>