I just found an interesting project, implementing a simple telnet-server on the ESP 8266. Not as secure as SSH, but if someone is clever enough to get into my wifi, the least I worry about is my SBC.
It's a german page, but the source should be unterstandable for everyone: https://www.nikolaus-lueneburg.de/2017/09/wifi-telnet-server-auf-dem-esp8266/
Hennes