ESP8266_RTOS_SDK Release v3.1

This is the list of changes since pre-release v3.1-rc1.

1. Important Notes

• V3.1 updated the bootloader to initialize SPI flash I/O mode and clock. So if you are using the V3.0 bootloader, and now upgrade to the new SDK, please disable the following configuration in the menuconfig:
  "Bootloader config ---> [ ] Bootloader init SPI flash"
• We moved some functions from IRAM to flash, including malloc and free fucntions, to save more memory. In this case, please do not read/write/erase flash during sniffer/promiscuous mode. You need to disable the sniffer/promiscuous mode at first, then read/write/erase flash.
• ESP8285 or ESP8266 + 1MB flash can use "Copy OTA Mode" for OTA, more details are in the examples/system/ota.

2. Changes in Functions and Features:

2.1 OTA Function

• Support "Copy OTA Mode" for ESP8285 or ESP8266 + 1MB flash instead of generating 2 OTA binaries. More details are in the examples/system/ota.
• Optimized the firmware upgrading from an old SDK (NonOS SDK, or RTOS SDK that earlier than v3.0) to a new SDK(IDF style RTOS SDK v3.0 or later versions). More details are in the document.
• Removed the old "ota" example, and renamed the "universal_ota" to be the new "ota" example instead.

2.2 AWS IoT

• Fixed the issue that AWS cannot work when using wolfSSL instead of mbedTLS.

2.3 Driver

• Fixed SPI example README.md wave image error.

2.4 NVS flash

• Fixed the issue that NVS stored data may be broke when power off at some abnormal circumstances.

3. Modified Documents

• Added document "fota-from-old-new.rst" that introduces how to update from an old SDK to a new SDK(IDF style) by OTA.
• Optimized README.md of example OTA.
• Updated wolfSSL license and copyright.
• Added document "General Notes".