add SinglePortPlugin to simpilify api

src/mesh/SinglePortPlugin.h

@@ -0,0 +1,24 @@
+#pragma once
+#include "MeshPlugin.h"
+
+/**
+ * Most plugins are only interested in sending/receving one particular portnum.  This baseclass simplifies that common
+ * case.
+ */
+class SinglePortPlugin : public MeshPlugin
+{
+  protected:
+    PortNum ourPortNum;
+
+  public:
+    /** Constructor
+     * name is for debugging output
+     */
+    SinglePortPlugin(const char *_name, PortNum _ourPortNum) : MeshPlugin(_name), ourPortNum(_ourPortNum) {}
+
+  protected:
+    /**
+     * @return true if you want to receive the specified portnum
+     */
+    virtual bool wantPortnum(PortNum p) { return p == ourPortNum; }
+};