BaseTools: Expression - change from series of if to elif

BaseTools: Expression - change from series of if to elif

since the first character of the string cannot be found by multiple if
statements, use elif to optomize the behavior.

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