graphics/nxmu/nxmu_start.c: Extend nxmu_start so that receives a display number as an argument. This makes it possible to have multiple instances of the NX server running on boards that support multiple displays. Also includes updates to boardctl() to accept display number paramters.

Documentation/NXGraphicsSubsystem.html

@@ -12,7 +12,7 @@
       <h1><big><font color="#3c34ec">
         <i>NX Graphics Subsystem</i>
       </font></big></h1>
-      <p>Last Updated: April 15, 2018</p>
+      <p>Last Updated: March 10, 2019</p>
     </td>
   </tr>
 </table>
@@ -956,7 +956,7 @@ struct nx_callback_s
 <ul><pre>
 #include &lt;nuttx/nx/nx.h&gt;
 
-int nxmu_start(void);
+int nxmu_start(int display, int plane);
 </pre></ul>
 <p>
   <b>Description:</b>
@@ -964,7 +964,12 @@ int nxmu_start(void);
 </p>
 <p>
   <b>Input Parameters:</b>
-  None
+  <ul><dl>
+    <dt><code>display</code>
+    <dd>The display number to be served by this new NXMU instance.
+    <dt><code>plane</code>
+    <dd>The plane number to use to get information about the display geometry and color format.
+  </dl></ul>
 </p>
 <p>
   <b>Returned Value:</b>