feat(ssl): Restructure ssl folder

Put mbedtls, axtls, openssl to ssl, also add kconfig to choose ssl library.
Default use mbedtls.

components/ssl/openssl/source/library/ssl_stack.c

@@ -0,0 +1,74 @@
+// Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "ssl_stack.h"
+#include "ssl_dbg.h"
+#include "ssl_port.h"
+
+#ifndef CONFIG_MIN_NODES
+    #define MIN_NODES 4
+#else
+    #define MIN_NODES CONFIG_MIN_NODES
+#endif
+
+/**
+ * @brief create a openssl stack object
+ */
+OPENSSL_STACK* OPENSSL_sk_new(OPENSSL_sk_compfunc c)
+{
+    OPENSSL_STACK *stack;
+    char **data;
+
+    stack = ssl_mem_zalloc(sizeof(OPENSSL_STACK));
+    if (!stack) {
+        SSL_DEBUG(SSL_STACK_ERROR_LEVEL, "no enough memory > (stack)");
+        goto no_mem1;
+    }
+
+    data = ssl_mem_zalloc(sizeof(*data) * MIN_NODES);
+    if (!data) {
+        SSL_DEBUG(SSL_STACK_ERROR_LEVEL, "no enough memory > (data)");
+        goto no_mem2;
+    }
+
+    stack->data = data;
+    stack->num_alloc = MIN_NODES;
+    stack->c = c;
+
+    return stack;
+
+no_mem2:
+    ssl_mem_free(stack);
+no_mem1:
+    return NULL;
+}
+
+/**
+ * @brief create a NULL function openssl stack object
+ */
+OPENSSL_STACK *OPENSSL_sk_new_null(void)
+{
+    return OPENSSL_sk_new((OPENSSL_sk_compfunc)NULL);
+}
+
+/**
+ * @brief free openssl stack object
+ */
+void OPENSSL_sk_free(OPENSSL_STACK *stack)
+{
+    SSL_ASSERT3(stack);
+
+    ssl_mem_free(stack->data);
+    ssl_mem_free(stack);
+}