To control the log level and the format of the log output of SDK libraries, a bunch of library-specific printf functions are realized which map the log output from SDK libraries to RIOT's log macros.
The modified version esptool.py from RTOS SDK that is required for flashing an image, is now placed in `dist/tools/esptool.py` and used directly from there. The advantage is that `esptool.py` hasn't to be installed explicitly anymore. Having RIOT is enough. The documentation is adapted accordingly. The oly prerequisite is that python and the pyserial module are installed.
To make the migration progress to the new RTOS SDK easier, the new toolchain was renamed to xtensa-esp8266-elf. This makes it possible to have the new and the old toolchain installed in parallel.
The documentation had to be changed due to the relation of module `esp_now` to `esp_wifi` module. In addition, a number of corrections have been made. In the case of documentation, it is impossible to do this in various commits.
