SecurityPkg: Make time based AuthVariable update atomic

SecurityPkg: Make time based AuthVariable update atomic

System may break during time based AuthVariable update, causing certdb inconsistent. 2 ways are used to ensure update atomic.
 1. Delete cert in certdb after variable is deleted
 2. Clean up certdb on variable initialization

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by: Yao Jiewen <jiewen.yao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17919 6f19259b-4bc3-4df7-8a09-765794883524