BaseTools: Allow string token identifier to use lower case letters

BaseTools: Allow string token identifier to use lower case letters

This patch is to align the code behavior with UNI spec that string token
identifier can use upper case and lower case letters.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Felix <Felixp@ami.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>