Debugging ESP8266 Boot Failures

TIL: If esptool can successfully communicate with an ESP8266, but the chip seems stone dead otherwise (i.e., no flashed programs work), it may be due to a wrong flash mode.

Debugging aid:

• The ESP8266 bootloader sends some debug output at 74880 baud after each reset.
• This baud rate is not supported by screen – miniterm.py (provided by python-serial) can handle it just fine, though.
• If it complains about a “csum err”, you probably flashed the wrong file / at the wrong address / used the wrong flash mode (there are differences e.g. between various NodeMCU / D1 mini shipments!)
• esptool -fm dout seems to be a safe (but slow) fallback