pc-bios/s390-ccw: always build network bootloader

pc-bios/s390-ccw: always build network bootloader

In the beginning, the network bootloader was considered experimental and
thus optional, but it is well established nowadays and configure always
checks for roms/SLOF before compiling pc-bios/s390-ccw.

Therefore, it makes sense to always build it together with the other
part of the s390-ccw bios.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>