BaseTool: Support different PCDs that refers to the same EFI variable.

BaseTool: Support different PCDs that refers to the same EFI variable.

V2:
Make the code of patch both compatible for Python2 and Python3.

V1:
If different PCDs refer to the same EFI variable, then do EFI variable
combination, according to the VariableOffset of different PCDS.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: ZhiqiangX Zhao <zhiqiangx.zhao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>