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 <72ed7ca0c1d67ded0b254900bd2112efa735a144@intel.com>
Cc: Maurice Ma <ffe085a909ec0c7dac9a0ba92c906627ea2355ed@intel.com>
Cc: Jiewen Yao <364a90bfebd1f362ebb7b48e4bf8ec010adef203@intel.com>
Cc: Giri P Mudusuru <25c53ebec15b492d82d7bf064153464c6e8fbfec@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <5c6bb72efe464613af94e84c05522a361b23844a@intel.com>
Reviewed-by: Jiewen Yao <364a90bfebd1f362ebb7b48e4bf8ec010adef203@intel.com>
Reviewed-by: Richard Thomaiyar <72ed7ca0c1d67ded0b254900bd2112efa735a144@intel.com>
Tested-by: Richard Thomaiyar <72ed7ca0c1d67ded0b254900bd2112efa735a144@intel.com>

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