OvmfPkg: control PXEv4 / PXEv6 boot support from the QEMU command line

OvmfPkg: control PXEv4 / PXEv6 boot support from the QEMU command line

Add a minimal, NULL class library called "PxeBcPcdProducerLib" for setting
the "PcdIPv4PXESupport" and "PcdIPv6PXESupport" PCDs of NetworkPkg, from
fw_cfg.

These PCDs control whether the UefiPxeBcDxe driver supports PXEv4 / PXEv6
boot. If a PXE version is disabled, the corresponding LoadFile protocol
instance is not produced by UefiPxeBcDxe, and so
EfiBootManagerRefreshAllBootOption() in UefiBootManagerLib does not
generate corresponding *new* boot options either. (Existent boot options
are not deleted.)

Hook the library into the UefiPxeBcDxe driver. (The driver is already
included from "NetworkComponents.dsc.inc", but we can list it again in the
DSC file, for providing <LibraryClasses> overrides.)

In OVMF, the PCDs could be set in PlatformPei too, but ArmVirtQemu does
not have fw_cfg access in the PEI phase. Hence a NULL class library that
can be linked into UefiPxeBcDxe.

When listing the PCDs under [PcdsDynamicDefault], stick with the DEC
default values.

QEMU switches:

  -fw_cfg name=opt/org.tianocore/IPv4PXESupport,string=[yn]

  -fw_cfg name=opt/org.tianocore/IPv6PXESupport,string=[yn]

The "opt/org.tianocore" prefix follows the "opt/RFQDN/" recommendation
from QEMU's "docs/specs/fw_cfg.txt".

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Per Sundstrom <per_sundstrom@yahoo.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2681
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200424075353.8489-6-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>