ESP_TLS: Restructuring esp_tls

1)Segregating mbedtls API into seperate file and cleaned esp_tls.c
2)Added support for wolfssl for CMake and make
3)Added support for debug_wolfssl (with menuconfig option)
4)Added info on wolfssl in ESP-TLS docs

docs/en/api-reference/protocols/esp_tls.rst

@@ -6,10 +6,13 @@ Overview
 
 The ESP-TLS component provides a simplified API interface for accessing the commonly used TLS functionality. 
 It supports common scenarios like CA certification validation, SNI, ALPN negotiation, non-blocking connection among others. 
-All the configuration can be specified in the esp_tls_cfg_t data structure. Once done, TLS communication can be conducted using the following APIs:
-* esp_tls_conn_new(): for opening a new TLS connection
-* esp_tls_conn_read/write(): for reading/writing from the connection
-* esp_tls_conn_delete(): for freeing up the connection
+All the configuration can be specified in the ``esp_tls_cfg_t`` data structure. Once done, TLS communication can be conducted using the following APIs:
+
+    * :cpp:func:`esp_tls_conn_new`: for opening a new TLS connection.
+    * :cpp:func:`esp_tls_conn_read`: for reading from the connection.
+    * :cpp:func:`esp_tls_conn_write`: for writing into the connection.
+    * :cpp:func:`esp_tls_conn_delete`: for freeing up the connection.
+
 Any application layer protocol like HTTP1, HTTP2 etc can be executed on top of this layer.                       
 
 Application Example
@@ -17,8 +20,75 @@ Application Example
 
 Simple HTTPS example that uses ESP-TLS to establish a secure socket connection: :example:`protocols/https_request`.
 
+Tree structure for ESP-TLS component
+-------------------------------------
+    .. highlight:: none
+
+    ::
+
+        ├── esp_tls.c
+        ├── esp_tls.h
+        ├── esp_tls_mbedtls.c
+        ├── esp_tls_wolfssl.c
+        └── private_include
+            ├── esp_tls_mbedtls.h
+            └── esp_tls_wolfssl.h
+
+The ESP-TLS  component has a file :component_file:`esp-tls/esp_tls.h` which contain the public API headers for the component. Internally ESP-TLS component uses one
+of the two SSL/TLS Libraries between mbedtls and wolfssl for its operation. API specific to mbedtls are present in :component_file:`esp-tls/private_include/esp_tls_mbedtls.h` and API
+specific to wolfssl are present in :component_file:`esp-tls/private_include/esp_tls_wolfssl.h`.
+
+Underlying SSL/TLS Library Options
+----------------------------------
+The ESP-TLS  component has an option to use mbedtls or wolfssl as their underlying SSL/TLS library. By default only mbedtls is available and is
+used, wolfssl SSL/TLS library is available publicly at https://github.com/espressif/esp-wolfssl. The repository provides wolfssl component in binary format, it
+also provides few examples which are useful for understanding the API. Please refer the repository README.md for
+information on licensing and other options. Please see below option for using wolfssl in your project.
+
+.. note::   `As the library options are internal to ESP-TLS, switching the libries will not change ESP-TLS specific code for a project.`
+
+How to use wolfssl with ESP-IDF
+-------------------------------
+There are two ways to use wolfssl in your project
+
+1) Directly add wolfssl as a component in your project with following three commands.::
+
+    (First change directory (cd) to your project directory)
+    mkdir components
+    cd components
+    git clone https://github.com/espressif/esp-wolfssl.git
+
+2) Add wolfssl as an extra component in your project.
+
+* Download wolfssl with::
+
+    git clone https://github.com/espressif/esp-wolfssl.git
+
+* Include  esp-wolfssl in ESP-IDF with setting EXTRA_COMPONENT_DIRS in CMakeLists.txt/Makefile of your project as done in `wolfssl/examples <https://github.com/espressif/esp-wolfssl/tree/master/examples>`_. For reference see Optional Project variables in :doc:`build-system.</api-guides/build-system>`
+
+After above steps, you will have option to choose wolfssl as underlying SSL/TLS library in configuration menu of your project as follows::
+
+    idf.py/make menuconfig -> ESP-TLS -> choose SSL/TLS Library -> mbedtls/wolfssl
+
+Comparison between mbedtls and wolfssl
+--------------------------------------
+The following table shows a typical comparison between wolfssl and mbedtls when :example:`protocols/https_request` example `(which has server authentication)` was run with both
+SSL/TLS libraries and with all respective configurations set to default.
+`(mbedtls IN_CONTENT length and OUT_CONTENT length were set to 16384 bytes and 4096 bytes respectively)`
+
+    +---------------------+------------+-----------+
+    | Property            |  Wolfssl   |  Mbedtls  |
+    +=====================+============+===========+
+    | Total Heap Consumed |  ~19 Kb    |  ~37 Kb   |
+    +---------------------+------------+-----------+
+    | Task Stack Used     |  ~2.2 Kb   |  ~3.6 Kb  |
+    +---------------------+------------+-----------+
+    |     Bin size        |  ~858 Kb   |  ~736 Kb  |
+    +---------------------+------------+-----------+
+
+.. note::    `These values are subject to change with change in configuration options and version of respective libraries`.
+
 API Reference
 -------------
 
 .. include:: /_build/inc/esp_tls.inc
-