kbuild: add dependency on kernel.release to the package targets

The binrpm-pkg target uses KERNELRELEASE when generated its .spec file.
When binrpm-pkg was the first build target run in a tree it generated the
.spec before kernel.release so the Version: tag in the .spec was empty.

I don't know if this is the best fix, but binrpm-pkg works when we
explicitly build kernel.release before descending into package-dir.

Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>