Add downcast methods to NetworkInterface

As we've introduced virtual inheritance to support EMACInterface, we can
no longer use C-style casts or static_cast to downcast from
NetworkInterface to more specific types. RTTI is disabled in the
toolchains, so dynamic_cast is unavailables.

Add virtual downcast methods to permit conversions to the 6 derived
classes. Probably only needed for EMACInterface, WiFiInterface and
EthInterface, but handles the set.

features/netsocket/CellularBase.h

@@ -101,6 +101,9 @@ public:
      */
     virtual const char *get_gateway() = 0;
 
+    virtual CellularBase *cellularBase() {
+        return this;
+    }
 };
 
 #endif //CELLULAR_BASE_H

features/netsocket/EMACInterface.h

@@ -150,6 +150,10 @@ public:
      */
     EMAC &get_emac() const { return _emac; }
 
+    virtual EMACInterface *emacInterface() {
+        return this;
+    }
+
 protected:
     /** Provide access to the underlying stack
      *

features/netsocket/EthInterface.h

@@ -29,6 +29,9 @@
  */
 class EthInterface : public virtual NetworkInterface
 {
+    virtual EthInterface *ethInterface() {
+        return this;
+    }
 };
 
 

features/netsocket/MeshInterface.h

@@ -29,6 +29,9 @@
  */
 class MeshInterface : public NetworkInterface
 {
+    virtual MeshInterface *meshInterface() {
+        return this;
+    }
 };
 
 

features/netsocket/NetworkInterface.h

@@ -21,9 +21,13 @@
 #include "netsocket/SocketAddress.h"
 #include "Callback.h"
 
-// Predeclared class
+// Predeclared classes
 class NetworkStack;
-
+class EthInterface;
+class WiFiInterface;
+class MeshInterface;
+class CellularBase;
+class EMACInterface;
 
 /** NetworkInterface class
  *
@@ -153,6 +157,30 @@ public:
      */
     virtual nsapi_error_t set_blocking(bool blocking);
 
+    /** Dynamic downcast to an EthInterface */
+    virtual EthInterface *ethInterface() {
+        return 0;
+    }
+
+    /** Dynamic downcast to a WiFiInterface */
+    virtual WiFiInterface *wifiInterface() {
+        return 0;
+    }
+
+    /** Dynamic downcast to a MeshInterface */
+    virtual MeshInterface *meshInterface() {
+        return 0;
+    }
+
+    /** Dynamic downcast to a CellularBase */
+    virtual CellularBase *cellularBase() {
+        return 0;
+    }
+
+    /** Dynamic downcast to an EMACInterface */
+    virtual EMACInterface *emacInterface() {
+        return 0;
+    }
 
 protected:
     friend class Socket;

features/netsocket/WiFiInterface.h

@@ -99,6 +99,10 @@ public:
      *                   negative on error see @a nsapi_error
      */
     virtual nsapi_size_or_error_t scan(WiFiAccessPoint *res, nsapi_size_t count) = 0;
+
+    virtual WiFiInterface *wifiInterface() {
+        return this;
+    }
 };
 
 #endif