BaseTools: create and use a standard shared variable for '*'

add a variable for the string '*' and then use it instead of lots of '*'

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 : Bob Feng <bob.c.feng@intel.com>