IntelFsp2Pkg: 41d739e breaks flat tree build

IntelFsp2Pkg: 41d739e breaks flat tree build

There may be no environment variable PACKAGES_PATH defined
in flat tree, then 41d739e breaks flat tree build.
This patch is to update GenCfgOpt.py to be compatible with
both flat tree and package path build.

Cc: Richard Thomaiyar <richard.marian.thomaiyar@intel.com>
Cc: Maurice Ma <maurice.ma@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Giri P Mudusuru <giri.p.mudusuru@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Richard Thomaiyar <richard.marian.thomaiyar@intel.com>
Tested-by: Richard Thomaiyar <richard.marian.thomaiyar@intel.com>