MdeModulePkg: Variable: Update DBT PCR[7] measure

MdeModulePkg: Variable: Update DBT PCR[7] measure

Measure DBT into PCR[7] when it is updated between initial measure
if present and not empty. by following TCG PC Client PFP 00.49
Previous patch for PCR[7] DBT part is overrode.
dc9bd6ed281fcba5358f3004632bdbda968be1e5

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>