ShellPkg/Bcfg: Add support for 'addp' command.
    
    Until now the 'addp' command has been handled in the same way as 'add'.
    Just copy the DevicePath starting from the Hard Drive node when 'addp'
    is used.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
    Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>