kbuild: rpm-pkg: refactor *rpm-pkg targets

Merge the similar build targets.

Also, make the output location consistent.

Previously, source packages were created in the build directory,
while binary packages under ~/rpmbuild/RPMS/.

Now, Kbuild creates the rpmbuild/ directory in the build directory,
and saves all packages under it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>