Use EEPROM_start to locate flash sector

Various changes were made in the ESP8266 core library that affect the way the EEPROM sector was defined. This broke the ESP_EEPROM library for some boards (e.g. Wemos Mini Pro with 16MB flash). A new define was introduced and the segment boundaries can be flexibly determined in ESP core version 3.1+.

If you are still using an older core library, then you need to use a ESP_EEPROM release <2.2.

Additional public constructor

Constructor made public to allow custom EEPROM segment. (N.B. html docs not regenerated)

Fixes for SPIFF & commit() inconsistency

Minor tweaks to make commit() return more sensible when it does nothing and fix for disappearance of SPIFFs

Bugfix and documentation

One issue fixed with percentUsed() and Doxygen documentation added to /docs directory

The documentation has been generated by Doxygen from comments added into the code.

v1.0.1

Initial release