kbuild: rpm-pkg: generate kernel.spec in rpmbuild/SPECS/

kernel.spec is the last piece that resides outside the rpmbuild/
directory. Move all the RPM-related files to rpmbuild/ consistently.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>