[RFC] PCD: Extended SKU support 1 - inheritance

["Zeng, Star" <star.zeng@intel.com>]

- Requirement
Simplify the PCDs configuring for multiple SKUs in DSC.


- Current limitation
Non-DEFAULT SKU could only derive from DEFAULT SKU, but could not derive from another non-DEFAULT SKU.
For example below, SkuA and SkuB could only derive from DEFAULT, but SkuB could not derive from SkuA.

[SkuIds]
  0 | DEFAULT
  1 | SkuA
  2 | SkuB


- Proposal: One non-DEFAULT SKU could be a derivative of another non-DEFAULT SKU.
This proposal only extends DSC [SkuIds] section syntax and the extension is optional.
This proposal keeps the backward compatibility with current SKU usage.
BaseTools update is needed to support the syntax extension, and no any change in PCD database and driver is required.

DSC syntax:
  [SkuIds]
    SkuValue|SkuName[|ParentSkuName]
      SkuValue: integer, 0 is reserved for DEFAULT SKU.
      SkuName: string
      ParentSkuName: string, optional, it is new introduced in this proposal and defines which SKU the PCD value will derive from for this SKU. The PCD value will derive from DEFAULT SKU for this SKU if the ParentSkuName is absent.


- Example: SkuB is a derivative of SkuA, but not a derivative of DEFAULT.

  [SkuIds]
    0 | DEFAULT
    1 | SkuA
    2 | SkuB | SkuA

  [PcdsDynamicDefault.Common.DEFAULT]
    gXXXPkgTokenSpaceGuid.PcdXXXSignature|"DEFAULT"
    gXXXPkgTokenSpaceGuid.PcdXXXConfig1|FALSE
    gXXXPkgTokenSpaceGuid.PcdXXXConfig2|FALSE
    gXXXPkgTokenSpaceGuid.PcdXXXConfig3|FALSE

  [PcdsDynamicDefault.Common.SkuA]
    gXXXPkgTokenSpaceGuid.PcdXXXSignature|"SkuA"
    gXXXPkgTokenSpaceGuid.PcdXXXConfig1|TRUE
    gXXXPkgTokenSpaceGuid.PcdXXXConfig2|TRUE
    # No need statement for PcdXXXConfig3 whose value will derive from DEFAULT SKU and be FLASE.
 
  [PcdsDynamicDefault.Common.SkuB]
    gXXXPkgTokenSpaceGuid.PcdXXXSignature|" SkuB"
    # No need statement for PcdXXXConfig1 and PcdXXXConfig2 whose values will derive from SkuA SKU and be TRUE.
    gXXXPkgTokenSpaceGuid.PcdXXXConfig3|TRUE