MdePkg and MdeModulePkg Pcd: Implement PCD Driver for External PCD Database and SKU enable Feature.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Bob C Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@14857 6f19259b-4bc3-4df7-8a09-765794883524