Handle os-prober and tftp path generation better.
    
    - Try to emit linux16/initrd16 and linuxefi/initrdefi when appropriate
      in 30_os-prober.
      Resolves: rhbz#1108296
    - If $fw_path doesn't work to find the config file, try $prefix as well
      Resolves: rhbz#1148652
    
    Signed-off-by: Peter Jones <pjones@redhat.com>