I have repaired by OS with help from Ray (thanks Ray). I tried many more times
to unbrick the ESP-12N but failed. Now I am back up.
With the latest firmware 2.1.8.(7) and the connection using the Ethernet adapter
ENC28J60 it hangs frequently. I cannot make it through a single watering cycle
without it hanging. When It is hung it will not respond to pings.
This is actually a much better situation for debugging. Previously it might
take more than a month to hang.
My current working theory is that the ESP processor does not respond fast enough
to prevent a ethernet buffer overflow. I will add some code to detect this
situation.