ESP8266_RTOS_SDK/components/esp8266

README

esp-idf compile and 'AR' user program as target library 'libmain.a', so we should not use
library which is named 'libmain.a'. We use 'libcore.a' instead of it.

As I konwn, you can do following:

1. rename libmain.a to libcore.a
2. change text 
        "*libmain.a:spi_flash.o(.literal .text .literal.* .text.*)"
   to
        "*libcore.a:spi_flash.o(.literal .text .literal.* .text.*)"
   in file 'eagle.app.v6.common.ld'