BaseTools: Extend the keyword "!include"/"!if" to case-insensitive

BaseTools: Extend the keyword "!include"/"!if" to case-insensitive

Extend the keyword "!include", "!if", etc to case-insensitive.
Current DSC parser already support it, while FDF parser only support
the lower case, so this patch add the support for FDF parser.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=1111
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>