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>
diff --git a/IntelFsp2Pkg/Tools/GenCfgOpt.py b/IntelFsp2Pkg/Tools/GenCfgOpt.py
index 0675b55..7658b7f 100644
--- a/IntelFsp2Pkg/Tools/GenCfgOpt.py
+++ b/IntelFsp2Pkg/Tools/GenCfgOpt.py
@@ -524,11 +524,14 @@
                                         IncludeFilePath = Match.group(1)

                                         IncludeFilePath = self.ExpandMacros(IncludeFilePath)

                                         PackagesPath = os.getenv("PACKAGES_PATH")

-                                        for PackagePath in PackagesPath.split(os.pathsep):

-                                            IncludeFilePathAbs = os.path.join(os.path.normpath(PackagePath), os.path.normpath(IncludeFilePath))

-                                            if os.path.exists(IncludeFilePathAbs):

-                                                IncludeDsc  = open(IncludeFilePathAbs, "r")

-                                                break

+                                        if PackagesPath:

+                                          for PackagePath in PackagesPath.split(os.pathsep):

+                                              IncludeFilePathAbs = os.path.join(os.path.normpath(PackagePath), os.path.normpath(IncludeFilePath))

+                                              if os.path.exists(IncludeFilePathAbs):

+                                                  IncludeDsc  = open(IncludeFilePathAbs, "r")

+                                                  break

+                                        else:

+                                          IncludeDsc  = open(IncludeFilePath, "r")

                                         if IncludeDsc == None:

                                             print("ERROR: Cannot open file '%s'" % IncludeFilePath)

                                             raise SystemExit