1. update support for 8MB & 16MB flash size
2. update compile ENV to generate image in 8MB & 16MB flash size map
3. update function "user_rf_cal_sector" in example code
1. add spi flash erase protect mechanism, boot and current runing user
bin can not been erased;
2. add memleak debug feature;
3. fix spi overlap issue;
4. fix call wifi_station_disconnect, disconnect event enter twice
issue;
5. fix crash when set opmode from station to softAP in scan done
callback;
6. modify spi flash erase/write flow, clear protect status if needed;
7. fix rf init data sector broken issue, add user_rf_cal_sector_set,
user application MUST have this function, refer to examples;
8. fix system parameter sector broken issue when frequently power
on/off;
9. fix the max value of os_timer_arm;
10. fix dns issue in some routers;
11. add sntp support;
12. update smartconfig to 2.5.4;
13. update open freedom to support send beacon packet;
14. fix seldom rf not work issue after external reset;
15. fix pwm issue;
16. fix status error got by wifi_station_get_connect_status;
17. fix assert in pp;
18. fix huawei X4 connect softAP issue;
19. optimize sleep strategy；
20. add vendor IE support;
21. update libphy.a to 1055;
22. add weak function wifi_set_backup_mac to let user set MAC;
23. fix bug of lwip and optimize socket mechanism;
24. update boot loader to v1.6;
25. update esp_init_data_default.bin
26. add mbedtls support；
27. fix other bugs;
1. Add libssc.a, simple serial console lib.
2. Add libspiffs.a, SPI file system.
3. Add libwps.a to support WPS.
4. Add libespconn.a, Espressif connection lib.
5. Add libespnow.a to support Espressif ESP-NOW.
6. Add libmesh.a, Espressif mesh.
7. Add libnopoll.a, websocket.
8. Add make_lib.sh in "third_party" folder.
9. Add modem-sleep & light-sleep supported.
10. Update libcirom.a to support float IO.
11. Update gen_misc.sh & gen_misc.bat.
12. Update header files, add comments in doxygen style.
13. Update libsmartconfig.a to version 2.5.2.
14. Update libssl.a.
15. Updates driver (PWM/UART/GPIO/SPI/Hardware timer).
16. Update open source codes of third_party.
17. Modify "ld" files, "dram0 len" should be 0x18000 in RTOS SDK.
18. Remove header files in extra_include, which are already in compile folder.
19. Other APIs sync from non-OS SDK, more details in documentation "20B-ESP8266__RTOS_SDK_API Reference".
20. Other optimization to make the SDK more stable.
