Add support of NSAPI_ICMP sockets in Nanostack

Author: YannCharbon

connectivity/nanostack/include/nanostack-interface/Nanostack.h

@@ -122,7 +122,7 @@ class Nanostack : public OnboardNetworkStack, private mbed::NonCopyable<Nanostac
      *  NSAPI_ERROR_NO_SOCKET is returned if no socket is available.
      *
      *  @param handle   Destination for the handle to a newly created socket
-     *  @param proto    Protocol of socket to open, NSAPI_TCP or NSAPI_UDP
+     *  @param proto    Protocol of socket to open, NSAPI_TCP, NSAPI_UDP or NSAPI_ICMP
      *  @return         0 on success, negative error code on failure
      */
     nsapi_error_t socket_open(void **handle, nsapi_protocol_t proto) override;

connectivity/nanostack/source/Nanostack.cpp

@@ -678,6 +678,8 @@ nsapi_error_t Nanostack::socket_open(void **handle, nsapi_protocol_t protocol)
         ns_proto = SOCKET_UDP;
     } else if (NSAPI_TCP == protocol) {
         ns_proto = SOCKET_TCP;
+    } else if (NSAPI_ICMP == protocol) {
+        ns_proto = SOCKET_ICMP;
     } else {
         MBED_ASSERT(false);
         return NSAPI_ERROR_UNSUPPORTED;