BaseTools: Add comments in StructurePcd DSC file.

BaseTools: Add comments in StructurePcd DSC file.

Currently VFR files have variables comments which will not be
added into StructurePcd.dsc file. Thus, it is not convenient for
developer to Modify Pcds. To solve this problem, The comments will
be modified to user friendly format and added after the corresponding
Pcd values in StructurePcd.dsc file.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Yuwei Chen <yuwei.chen@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>