OvmfPkg: Add a build flag to select ISA driver stack

OvmfPkg: Add a build flag to select ISA driver stack

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1495

This commit will add a static build flag 'USE_LEGACY_ISA_STACK' to select
the ISA driver stack.

If the flag is set to TRUE, the below driver stack will be used:
  PcAtChipsetPkg/IsaAcpiDxe/IsaAcpi.inf
  IntelFrameworkModulePkg/Bus/Isa/IsaBusDxe/IsaBusDxe.inf
  IntelFrameworkModulePkg/Bus/Isa/IsaSerialDxe/IsaSerialDxe.inf
  IntelFrameworkModulePkg/Bus/Isa/Ps2KeyboardDxe/Ps2keyboardDxe.inf

If the flag is set to FALSE, the below driver stack will be used:
  OvmfPkg/SioBusDxe/SioBusDxe.inf
  MdeModulePkg/Bus/Pci/PciSioSerialDxe/PciSioSerialDxe.inf
  MdeModulePkg/Bus/Isa/Ps2KeyboardDxe/Ps2KeyboardDxe.inf

The default value is set to FALSE in OVMF DSC files.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ray Ni <ray.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Anthony PERARD <anthony.perard@citrix.com>