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Description
Arduino IDE 3rd party Hardware specification section "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 #5770