REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1323 Merge EmuVariable and Real variable driver. Add macro NV_STORAGE_VARIABLE_BASE. Not get NV PCD in VariableWriteServiceInitialize, but in FtwNotificationEvent/SmmFtwNotificationEvent, then VariableWriteServiceInitialize could be not aware the NV storage is real or emulated. This patch prepares for adding emulated variable NV mode support in VariableRuntimeDxe. Cc: Jian J Wang <jian.j.wang@intel.com> Cc: Hao Wu <hao.a.wu@intel.com> Contributed-under: TianoCore Contribution Agreement 1.1 Signed-off-by: Star Zeng <star.zeng@intel.com> Reviewed-by: Jian J Wang <jian.j.wang@intel.com> Reviewed-by: Hao Wu <hao.a.wu@intel.com> Tested-by: Julien Grall <julien.grall@arm.com> Acked-by: Julien Grall <julien.grall@arm.com>