Use "packout" reboot reason for the firstboot shutdown

Use "packout" to trigger serial number / IMEI displayed in bootloader
for in-boxing process.

Bug: 112744852
Change-Id: I5e068c26915251a2aeb4d7a6b87ad7b949538ae7
diff --git a/init.firstboot.sh b/init.firstboot.sh
index 05a1b85..f520405 100755
--- a/init.firstboot.sh
+++ b/init.firstboot.sh
@@ -15,4 +15,4 @@
 done;
 
 # Shutdown
-setprop sys.powerctl shutdown
+setprop sys.powerctl reboot,packout