Implement READ/WRITE to EEPROM

[AxxAxx]

By utilizing EEPROM one could for example:

• Remember SET Temperature - Save the current SET Temperature and automatically use that same value next time AxxSolder is powered on.
• Adjust EMERGENCY_SLEEP_TIME - Set the time for when AxxSolder will automatically go into sleep mode, currently set to 30 min.
• Set tuning parameters - allow the user to calibrate the temperature of AxxSolder and save the settings.

[AxxAxx]

Closed - solved