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;
