Feature

• Update to SDK version 3.0.4 and AT version 1.7.4.0
• feat(pp): Used beacon interval to calibrate timer to improve accuracy
• feat(net80211): Allowed ESP8266 to connect to AP without basic rate
• feat(system): revise flash map automatically when the firmware and flash map do not match.
• feat(at): Add at_nano demo to support 1MB flash

Bug Fix

• fix(net80211): Fixed some connection failure issue during 11b with only AP mode
• fix(net80211): Fixed the memory leak problem when ESP8266 acts as AP and station is sending illegal messages
• fix(system): Fixed probable loss of system parameter if power down when erasing or writing the flash
• fix(lwip): Fixed crash issue when SNTP is stopped
• fix(at): Fixed potential low risk of scan failure using the AT command to scan specified SSID