system: show what the console command printed

The console tab posts to /cmd and throws the response away, so the page stayed
silent whether the command worked, was refused or was just misspelt. Now that
/cmd answers with the command's output, the page keeps a running transcript:
each command echoed after a prompt, then whatever it printed.

The box is a pre element filled through textContent, so nothing the device
prints is taken for markup. It scrolls to the newest line and the input clears
after a send.

html/system.html

@@ -88,6 +88,8 @@
       <input style="width:20%;" class="action" id="cmd_sub" onclick="cmdSub();" type="button" data-i18n="sys_send_cmd" value="Send Command"><br/>
       <br/><br/> 
       <span data-i18n="sys_console_warn">Be careful when entering console commands, you can lock yourself out!</span><br/>
+      <br/>
+      <pre id="console_out" style="height:22em;overflow:auto;white-space:pre-wrap;border:1px solid;padding:0.5em;"></pre>
 
     </div>