feat(example): Add openssl server demo

internal: 9c54c474

examples/openssl_server/README.md

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+1. Run ./gencrt.sh or if you have your own certificate, move to the openssl_server directory, the name is ca.crt,server.crt, server.key, client.crt and client.key.
+
+    The server.crt and client.crt was generate by the same ca.crt in ./gencrt.sh.
+
+    Server side needs ca.crt(to verify client.crt), server.crt, server.key
+
+    Client side needs ca.crt(to verify server.crt), client.crt, client.key
+
+    If you have two ca.crt to generate server.crt and client.crt respectively, client1.crt is generate by ca1.crt and client1.key, server2.crt is generate by ca2.crt and server2.key:
+
+    Client side needs ca2.crt, client1.crt, client1.key.
+
+    Server side needs ca1.crt, server2.crt, server2.key.
+
+    Rename ca1.crt server2.crt server2.key to ca.crt server.crt server.key and run ./genheader.sh.
+
+    Use ca2.crt in openssl s_client -CAfile option.
+
+2. Run ./genheader.sh.
+
+3. Modify thease two lines in file user_config.h to your local Wi-Fi SSID and Password.
+
+    ```#define SSID "HUAWEI001"```
+
+    ```#define PASSWORD ""```
+
+4. Make sure that the computer and ESP8266 are in the same local area network.
+
+5. Run ./gen_misc.sh.
+
+6. Download bin file to ESP8266.
+
+    Find server ip address in ESP8266 UART log: ip:192.168.3.6,mask:255.255.255.0,gw:192.168.3.1.
+
+7. Run openssl s_client -CAfile ca.crt -cert client.crt -key client.key -verify 1 -tls1_1 -host 192.168.3.6 -port 443.
+
+
+**ATTENTION**
+
+**1. Make sure the free heap size larger than 30K.**
+
+**2. Make sure the private key length larger than 2048.**
+
+**3. Make sure the fragment size range is between 2048 and 8192.**