This is a fake platformio package to make it work on OpenBSD. It is inspired by https://github.com/trombik/platformio-freebsd-toolchain-xtensa/.
$ pkg_add arduino-esp8266
Go to ~/.platformio/packages and clone this repository. It is important to name the cloned directory tool-esptool.
$ cd ~/.platformio/packages
$ git clone https://github.com/lotherk/platformio-openbsd-tool-esptool tool-esptool
$ cd tool-esptool
$ ./init.sh