BaseTools: Fix the Keyword error for <ExtendedFvEntry> in FDF File

BaseTools: Fix the Keyword error for <ExtendedFvEntry> in FDF File

current in FDF spec 3.6 [FV] section it use "FV_EXT_ENTRY_TYPE" as
Keyword for <ExtendedFvEntry>, while in the code it use "FV_EXT_ENTRY".
To keep compatibility, this patch support both keyword in the code
first.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>