kbuild: rpm: prompt to use "rpm-pkg" if "rpm" target is used

The "rpm" has been kept for backward compatibility since pre-git era.
I am planning to remove it after the Linux 4.18 release.  Annouce the
end of the support, prompting to use "rpm-pkg" instead.

If you use "rpm", it will work like "rpm-pkg", but warning messages
will be displayed as follows:

  WARNING: "rpm" target will be removed after Linux 4.18
           Please use "rpm-pkg" instead.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>