[pprz center] possibility to launch global commans from tools using $ prefix

conf/control_panel_example.xml

@@ -71,6 +71,8 @@
       <arg flag="-b" variable="ivy_bus"/>
     </program>
 
+    <program name="Http Server" command="$python -m SimpleHTTPServer 8889"/>
+
     <program name="Plot Meteo Profile" command="sw/logalizer/plotprofile"/>
 
     <program name="Weather Station" command="sw/ground_segment/misc/davis2ivy">

sw/supervision/pc_control_panel.ml

@@ -42,6 +42,8 @@ let program_command = fun x ->
     let cmd = ExtXml.attrib xml "command" in
     if cmd.[0] = '/' then
       cmd
+    else if cmd.[0] = '$' then
+      String.sub cmd 1 ((String.length cmd) - 1)
     else
       Env.paparazzi_src // cmd
   with Not_found ->