MdeModulePkg: Update PCD driver to support the optimized PcdDataBase

MdeModulePkg: Update PCD driver to support the optimized PcdDataBase

https://bugzilla.tianocore.org/show_bug.cgi?id=546
BaseTools will generate the optimized PCD database to save the image size
at build time for multiple SKUs. The optimized PCD database layout will be like
below, the PCD database will be composed of the full default SKU data
(PCD_DATABASE_INIT) and the non-default SKU delta data(PCD_DATABASE_SKU_DELTA).
PCD driver will build HOB to store the full default SKU data, and patch HOB
data based on non-default SKU delta data for the SKU set by SetSku(),
it can save memory resource at boot time.

//
// PCD database layout:
// +---------------------------------+
// | PCD_DATABASE_INIT (DEFAULT SKU) |
// +---------------------------------+
// | PCD_DATABASE_SKU_DELTA (SKU A)  |
// +---------------------------------+
// | PCD_DATABASE_SKU_DELTA (SKU B)  |
// +---------------------------------+
// | ......                          |
// +---------------------------------+
//

BaseTools, PCD database and driver updates are needed for this proposal.
For single SKU (default) case, this proposal is expected to have no impact.
For multi-SKU case, PCD database format will be changed.
So, PcdDataBase Version is also updated from 6 to 7.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>