Fix panic due to after.sh prematurely quitting on shutdown

The sysrq shutdown trigger takes some time to fully shut down the system,
during which init is expected to continue running. Since after.sh is the
last step in our init, if it quits before shutdown is complete, Linux will
panic with "Attempted to kill init".

Add an infinite loop after shutdown is issued via sysrq to prevent this.

steps/improve/after.sh

@@ -21,4 +21,5 @@ if [ "${CHROOT}" = False ]; then
     echo u > /proc/sysrq-trigger
     mount -o remount,ro /
     echo o > /proc/sysrq-trigger # power off
+    while true; do sleep 1; done
 fi