Make [-D Macros] as optional argument for GenCfgOpt

Cc: Maurice Ma <maurice.ma@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Richard Thomaiyar <richard.marian.thomaiyar@intel.com>
Reviewed-by: Maurice Ma <maurice.ma@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>