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>

scripts/package/Makefile

@@ -45,7 +45,7 @@ rm -f $(objtree)/.scmversion
 
 # rpm-pkg
 # ---------------------------------------------------------------------------
-rpm-pkg rpm: FORCE
+rpm-pkg: FORCE
 	$(MAKE) clean
 	$(CONFIG_SHELL) $(MKSPEC) >$(objtree)/kernel.spec
 	$(call cmd,src_tar,$(KERNELPATH),kernel.spec)