esp8266 Arduino package can't be used as referenced package

Arduino IDE 3rd party Hardware specification section ["Referencing another core, variant or tool"](https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5-3rd-party-Hardware-specification#referencing-another-core-variant-or-tool) specifies that a boards package can be referred from custom hardware definitions or other boards packages.

esp8266 package can't be used now as referred, because platform.txt file entries use variable {runtime.platform.path}, which points to the folder of selected board's boards.txt file. In case of a referring definition it is not the right folder to access sdk and tools.

solved in PR https://github.com/esp8266/Arduino/pull/5770