OvmfPkg: QemuBootOrderLib: expose QEMU's "-boot menu=on[,splash-time=N]"
    
    The QEMU command line option
    
      -boot menu=on
    
    is meant to have the guest firmware wait for a firmware-specific interval
    for the user to enter the boot menu. During the wait, the user can opt to
    enter the boot menu, or interrupt the wait and proceed to booting at once.
    If the wait interval elapses, the firmware should boot as it normally
    would.
    
    The QEMU command line option
    
      -boot menu=on,splash-time=N
    
    means the same, except the firmware should wait for cca. N milliseconds
    instead of a firmware-specific interval.
    
    We can approximate this behavior quite well for edk2's virtual platforms
    because the Intel BDS front page already supports a progress bar, with
    semantics similar to the above. Let's distill the fw_cfg bits underlying
    "-boot menu=on,splash-time=N" for the BDS policies, in the form of a
    timeout value they can pass to Intel's PlatformBdsEnterFrontPage().
    
    If the boot menu is not requested, we return
    "gEfiIntelFrameworkModulePkgTokenSpaceGuid.PcdPlatformBootTimeOut", which
    is what the virtual platforms use right now.
    
    If the boot menu is requested without specifying the timeout, we return
    the same PCD, unless it would cause us to skip the boot menu at once. In
    the latter case, we return 3 seconds (as an approximation of the 2500 ms
    SeaBIOS default.)
    
    RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1170507
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>