OvmfPkg/QemuFwCfgDxeLib: allow UEFI_DRIVER modules

OvmfPkg/QemuFwCfgDxeLib: allow UEFI_DRIVER modules

We don't distribute UEFI_DRIVER modules stand-alone that were built as
part of an OVMF platform. OVMF's UEFI_DRIVERs are allowed to inherit
platform dependencies.

By enabling UEFI_DRIVERs to consume QemuFwCfgDxeLib, we can hook
fw_cfg-based NULL class libraries into UEFI drivers, e.g. in order to set
dynamic PCDs.

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: <20200424075353.8489-5-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>