Implement the NSAPI_KEEPINTVL in the LWIPInterface

net/LWIPInterface/LWIPInterface.cpp

@@ -555,6 +555,13 @@ int LWIPInterface::setsockopt(void *handle, int level, int optname, const void *
             }
 
             s->tcp->so_options |= SOF_KEEPALIVE;
+            return 0;
+
+        case NSAPI_KEEPINTVL:
+            if (optlen != sizeof(int) || s->proto != NSAPI_TCP) {
+                return NSAPI_ERROR_UNSUPPORTED;
+            }
+
             s->tcp->keep_intvl = *(int*)optval;
             return 0;