Update to SDK 1.4

[grahamehorner]

update to use esp8266 sdk v1.4.0 - esp_iot_sdk_v1.4.0_15_09_18.zip and provide a method of obtaining the sdk used for the build of core libraries.

[igrr]

There is a branch based on SDK 1.4: https://github.com/igrr/Arduino/tree/sdk-1.4
However currently there are some issues with 1.4 related to AP mode and sleep modes, so I'm not upgrading the core yet.

Regarding the second part, i'm not sure i understand what you mean by 'provide a method of obtaining the sdk used for the build of core libraries'. SDK is in this repository, https://github.com/esp8266/Arduino/tree/esp8266/hardware/esp8266com/esp8266/tools/sdk. You can see which version is used by looking at version file. If you need to get this information at run time, there is a function in the SDK which returns version number. See SDK docs for details.

[igrr]

Updated to 1.5 (#1102)