kernel-source: include the kernel signature file
    
    We assume that the upstream tarball is used for released kernels.
    
    Then we can also include the signature file and keyring in the
    kernel-source src.rpm.
    
    Because of mkspec code limitation exclude the signature and keyring from
    binary packages always - mkspec does not parse spec conditionals.
    
    Signed-off-by: Michal Suchanek <msuchanek@suse.de>