BaseTools: Fixed Pcd value override issue.

BaseTools: Fixed Pcd value override issue.

1. Handle the Pcd maxsize issue for the case
that the length of Pcd value from CommandLine
bigger that its maxsize

2. The Pcd value override in commandline.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>