[comboot] Implement INT22,0x000c

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/syslinux/comboot_call.c

 		}
 		break;
 
+	case 0x000C: /* Perform final cleanup */
+		shutdown_boot();
+		break;
+
 	case 0x000E: /* Get configuration file name */
 		/* FIXME: stub */
 		ix86->segs.es = rm_ds;