BaseTools: Move OverrideAttribs to OptRomInfStatement.py

Move "class OverrideAttribs" to OptRomInfStatement.py to remove
"import OptionRom" which may form a circular import:
GenFds.OptionRom => GenFds.OptRomInfStatement => GenFds.OptionRom

Contributed-under: TianoCore Contribution Agreement 1.1
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>

BaseTools/Source/Python/GenFds/FdfParser.py

@@ -4493,7 +4493,7 @@ class FdfParser:
     #
     def __GetOptRomOverrides(self, Obj):
         if self.__IsToken('{'):
-            Overrides = OptionRom.OverrideAttribs()
+            Overrides = OptRomInfStatement.OverrideAttribs()
             while True:
                 if self.__IsKeyword( "PCI_VENDOR_ID"):
                     if not self.__IsToken( "="):