Merge pull request #8921 from gschorcht/esp8266

boards/common/esp8266/Makefile

boards/common/esp8266/Makefile.dep

boards/common/esp8266/Makefile.features

boards/common/esp8266/Makefile.include

boards/common/esp8266/board_common.c

boards/common/esp8266/doc.txt

boards/common/esp8266/include/board_common.h

boards/common/esp8266/include/board_modules.h

boards/common/esp8266/include/periph_conf_common.h

boards/esp8266-esp-12x/Makefile

boards/esp8266-esp-12x/Makefile.dep

boards/esp8266-esp-12x/Makefile.features

boards/esp8266-esp-12x/Makefile.include

boards/esp8266-esp-12x/doc.txt

boards/esp8266-esp-12x/include/arduino_board.h

boards/esp8266-esp-12x/include/board.h

boards/esp8266-esp-12x/include/gpio_params.h

boards/esp8266-esp-12x/include/periph_conf.h

boards/esp8266-olimex-mod/Makefile

boards/esp8266-olimex-mod/Makefile.dep