OvmfPkg: PlatformDxe: connect ExtractConfig() to platform data

OvmfPkg: PlatformDxe: connect ExtractConfig() to platform data

Establish the full stack of conversions in retrieving the platform
configuration:

    MultiConfigAltResp       -- form engine / HII communication
            ^
            |
     [BlockToConfig]
            |
     MAIN_FORM_STATE         -- binary representation of form/widget state
            ^
            |
[PlatformConfigToFormState]
            |
     PLATFORM_CONFIG         -- accessible to DXE and UEFI drivers
            ^
            |
   [PlatformConfigLoad]
            |
  UEFI non-volatile variable -- accessible to external utilities

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15374 6f19259b-4bc3-4df7-8a09-765794883524