OvmfPkg: PlatformBdsLib: signal End-of-Dxe event group

(Paraphrasing git commit 9cd7d3c5 / SVN r17713:)

Currently, OvmfPkg fails to signal the End-of-Dxe event group when
entering the BDS phase, which results in some loss of functionality, eg.
variable reclaim in the variable driver, and the memory region splitting
in the DXE core that belongs to the properties table feature specified in
UEFI-2.5.

As discussed on the edk2-devel mailing list here:

http://thread.gmane.org/gmane.comp.bios.tianocore.devel/16088/focus=16109

it is up to the platform BDS to signal End-of-Dxe, since there may be
platform specific ordering constraints with respect to the signalling of
the event that are difficult to honor at the generic level.

(OvmfPkg specifics:)

(1) In OvmfPkg, we can't signal End-of-Dxe before PCI enumeration
    completes. According to the previous patch, that would trigger
    OvmfPkg/AcpiS3SaveDxe to save S3 state *before* the following chain of
    action happened:

    - PCI enumeration completes
    - ACPI tables are installed by OvmfPkg/AcpiPlatformDxe
    - the FACS table becomes available

    Since OvmfPkg/AcpiS3SaveDxe can only save S3 state once the FACS table
    is available, we must delay the End-of-Dxe signal until after PCI
    enumeration completes (ie. root bridges are connected).

(2) Pre-patch, S3Ready() in OvmfPkg/AcpiS3SaveDxe is entered from
    BdsLibBootViaBootOption()
    [IntelFrameworkModulePkg/Library/GenericBdsLib/BdsBoot.c].

    After the patch, we enter S3Ready() earlier than that, by signaling
    End-of-Dxe in PlatformBdsPolicyBehavior(). The timing / location of
    this new call is correct as well, and the original call (that now
    becomes the chronologically second call) becomes a no-op: S3Ready() is
    protected against 2nd and later entries.

Cc: Jordan Justen <jordan.l.justen@intel.com>
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@18035 6f19259b-4bc3-4df7-8a09-765794883524

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