firstboot-complete: tell zipl to run

zipl records need to be updated, because ignition.firstboot is burned
into target disk during coreos-installer

As a short-term solution for:
#84

Depends on:
ibm-s390-linux/s390-tools#71
ibm-s390-linux/s390-tools#74

Related:
coreos/coreos-installer#61
coreos/coreos-assembler#780
(cherry picked from commit 38af701)

systemd/ignition-firstboot-complete.service

@@ -17,7 +17,13 @@ RemainAfterExit=yes
 # detected this file. Fail if we are unable to remove it, rather than risking
 # rerunning Ignition at next boot.
 MountFlags=slave
-ExecStart=/bin/sh -c 'mount -o remount,rw /boot && rm /boot/ignition.firstboot'
+# It is better to have a separate script to do this but it might be polluting
+# the target system with some script in i.e. /usr/sbin/firstboot-complete
+# The retval code is still respected with having this if-else block.
+ExecStart=/bin/sh -c \
+	'mount -o remount,rw /boot && \
+	if [[ $(uname -m) = s390x ]]; then zipl; fi && \
+	rm /boot/ignition.firstboot'
 
 [Install]
 # Part of basic.target so this happens early on in firstboot