SecurityPkg/DxeImageVerificationLib: refactor db/dbx fetching code (CVE-2019-14575)

SecurityPkg/DxeImageVerificationLib: refactor db/dbx fetching code (CVE-2019-14575)

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1608

The dbx fetching code inside the while/for-loop causes code hard to
understand. Since there's no need to get dbx more than once, this patch
simplify the code logic by moving related code to be outside the while-
loop. db fetching code is also refined accordingly to reduce the indent
level of code.

More comments are also added or refined to explain more details.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>