BaseTools: Latter full value should overwrite the former field value.

BaseTools: Latter full value should overwrite the former field value.

For structure Pcd, the latter full assign value in commandLine should
override the former field assign value. For example in commandLine,
build --pcd Token.pcd.field="haha" --pcd Token.pcd=H"{0x01,0x02}",
the former field value "haha" will be ignored and overwrite by the latter
full value "{0x01,0x02}".

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: ZhiqiangX Zhao <zhiqiangx.zhao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>