Use port=0 for randomization

Network stack will randomize the address once it's being set to port 0.
NOTE! Dependency on mbed OS 5.3, nanostack does the port randomization starting with mbedOS 5.3.

Author: Janne Kiiskilä

simpleclient.h

@@ -124,7 +124,7 @@ class MbedClient: public M2MInterfaceObserver {
                           void *handler=NULL) {
     // Randomizing listening port for Certificate mode connectivity
     _server_address = server_address;
-    uint16_t port = rand() % 65535 + 12345;
+    uint16_t port = 0; // Network interface will randomize with port 0
 
     // create mDS interface object, this is the base object everything else attaches to
     _interface = M2MInterfaceFactory::create_interface(*this,