SecurityPkg: DxeImageVerificationLib: Update PCR[7] measure logic

SecurityPkg: DxeImageVerificationLib: Update PCR[7] measure logic

Update PCR[7] measure logic according to TCG PC Client PFP 00.37.
Only entries in DB that is used for image authentication need to be
measured.
http://www.trustedcomputinggroup.org/wp-content/uploads/PC-ClientSpecific_Platform_Profile_for_TPM_2p0_Systems_v21.pdf

Cc: Star Zeng <star.zeng@intel.com>
Cc: Yao Jiewen <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Yao Jiewen <jiewen.yao@intel.com>