pc-bios/s390-ccw: zero out bss section

The QEMU ELF loader does not zero the bss segment.
This resulted in several bugs, e.g. see

commit 5d739a4787a5 (s390-ccw.img: Fix sporadic errors with ccw boot image - initialize css)
commit 6a40fa2669d3 (s390-ccw.img: Initialize next_idx)
commit 8775d91a0f42 (pc-bios/s390-ccw: Fix problem with invalid virtio-scsi LUN when rebooting)

Let's fix this once and forever by letting the BIOS zero the bss itself.

Suggested-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20171122142627.73170-3-borntraeger@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
