Upgrade path for ESP WiFi V1 users

[glynhudson]

There is a large users base of OpenEVSE users running @chris1howell WiFi EPS V1 FW:

https://github.com/OpenEVSE/ESP8266_WiFi_v1.x

It would be very useful if users could update from V1 > V2.

• V1 has a working HTTP update
• It's acceptable IMO for the config to be lost, therefore the user would need to re-enter their WiFi password and Emoncms API key. Do you agree @chris1howell ?
• It's acceptable for the update to be a two-part process if required. Most OpenEVSE users should be happy following update instructions.
• Having to resort to serial update should be avoided if possible

[jeremypoulter]

Needs testing but should be fixed after #87 and #90