added power off option in pxe menu

data/apache/files/boot.ipxe

@@ -39,7 +39,9 @@ item --gap -- Misc
 item chain-online Chain on rthoni.com
 item config Config
 item shell Shell
+item --gap -- Power
 item reboot Reboot
+item poweroff Poweroff
 choose choice && goto ${choice}
 
 <?php
@@ -60,6 +62,9 @@ shell
 :reboot
 reboot
 
+:poweroff
+poweroff
+
 :boot-install
 set unix install/${install}
 set options vga=788 -- quiet