apps: Add ability to do a clean reboot

Allow a clean shutdown to end in either power off or reboot. Add a
new event SYS_REBOOT to signal it and sys_reboot() to trigger the
event. SYS_REBOOT signals a reboot request and should be listened
for alongside SYS_POWEROFF events.

Change-Id: I99ba7fb5feed2bb5a0a40a274e8466ad74fe3a43

apps/iap/iap-core.c

@@ -364,6 +364,7 @@ static void iap_thread(void)
 
             /* Handle poweroff message */
             case SYS_POWEROFF:
+            case SYS_REBOOT:
             {
                 iap_shutdown = true;
                 break;