Use string pointer instead string buffer to avoid string copy operation.

Use string pointer instead string buffer to avoid string copy operation.
Use CopyMem() to guarantee the NULL terminal will always be appended to the destination string.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu, Siyuan <siyuan.fu@intel.com>
Reviewed-by: Yao, Jiewen <jiewen.yao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15810 6f19259b-4bc3-4df7-8a09-765794883524