BaseTools: Add two new sections for PCD in the build report

BaseTools: Add two new sections for PCD in the build report

Build Spec updated to add two new sections for PCD in the build report.
1.Conditional directives section:If the DSC or FDF file contains
conditional directive statements.
2.Unused PCDs section: If the DSC or FDF file define values for PCDs that
are not used by any module and are not used in conditional directive
statements.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>