s390x: Add linemode buffer to fix newline on every print

Linemode seems to add a newline for each sent message which makes
reading rather hard. Hence we add a small buffer and only print if
it's full or a newline is encountered.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190920112345.2359-1-frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>