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

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

Port the DSC file changes from the similarly titled OvmfPkg patch in this
series to ArmVirtPkg.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Leif Lindholm <leif@nuviainc.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: <20200424075353.8489-8-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>