v2.0.0

1. Updated libphy.a to 1055, fixed an issue of large current in Light-sleep.
2. Updated AT+ to 1.3.0:
   2.1 Added Light-sleep wakeup command, AT+WAKEUPGPIO;
   2.2 Fixed abnormal AT+CWDHCPS IP allocation;
   2.3 Added at_sdio demo under example directory.
3. Fixed probable system default when calling cur and def interfaces.
4. Fixed the issue of high current in Deep-sleep mode without disabling SPI first.
5. Fixed an issue where the SDK would crash when switching from AP to Station mode by calling wifi_set_phy_mode.
6. Updated secondary boot loader to v1.6:
   6.1 Supports AT + based on SDIO;
   6.2 Supports entering testing mode through GPIO.
7. Added support for MXIC Flash QIO mode.
8. Fixed exception caused during TCP handshake and retransmission.
9. Fixed issues in ESP-NOW.
10. Added ESP-PAIR (Simple-Pair) feature, for APIs please refer to ESP8266 SDK API Guide.
11. wpa2-enterprise time function derivation and time check disable can be set by users.
12. Support for PEAP/TTLS in wpa2-enterprise, for APIs please refer to ESP8266 SDK API Guide.
13. Added mqtt demos under examples directory.
14. Other issue fixes.
15. Resolved an issue that RF_Init_data sector may be broken in stress test. Provided a function user_rf_cal_sector_set which has to be added in application by software developer. More details about user_rf_cal_sector_set refer to documentation "2C-ESP8266__SDK__API Guide". (Resolved in ESP8266_NONOS_SDK_V1.5.4.1)
16. Fix a potential risk that will cause rf not work after external reset. (Resolved in ESP8266_NONOS_SDK_V1.5.4.1)
17. Add SDIO AT support. (Resolved in ESP8266_NONOS_SDK_V1.5.4.1)
18. Fix a potential bug in espconn. (Resolved in ESP8266_NONOS_SDK_V1.5.4.1)