kbuild: rpm-pkg: build binary packages from source rpm

The build rules of rpm-pkg and srcrpm-pkg are almost the same.
Remove the code duplication.

Change rpm-pkg to build binary packages from the source package generated
by srcrpm-pkg.

This changes the output directory of the srpm generated by 'make rpm-pkg'
because srcrpm-pkg overrides _srcrpmdir.

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