[S390] early: Fix possible overlapping data buffer

This patch fixed bugzilla #12965:
https://bugzilla.kernel.org/show_bug.cgi?id=12965

The original code contains some inproper use of sprintf
function where a buffer is used both as input string
as well as output string. It should remember the written
bytes in the previous and use that as the offset for
later writing. Also replace sprintf with snprintf.

Signed-off-by: Chen Liu <chenliu@asset.uwaterloo.ca>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>