Change cooling fan setpoint from 50c to 40c

[blacktigersoftware]

First, why 50c? For people with a MK10 type dual extruder (ie FlashForge Creator Pro), this improves heat dissipation for single extruder prints.

Notice that both thermal tubes are fully extended through an aluminum crossbar and the extruder motors are flush against said crossbar. When doing a single extruder print, heat comes up from the thermal heat-break tube, into the crossbar and back down the other thermal heat-break tube, into the other heating block and can raise the temperature up to 50c, and then the second fan kicks on. The second, unused motor provides a heat capacitor, which cools down from the second fan. I was able to take a temperature measurement from the end of the aluminum crossbar with a probe and confirmed it got to 50c and then held pretty steady.

By engaging the second cooling fan and trying to keep the temperature down will reduce PTFE tube jamming, allowing for longer prints at high temperatures, think 250c for ABS. Yes, the tube start to break down, but they should be replaced per regular maintenance if you do ABS or PETG.

I've successfully changed my EEPROM setting (manually, by rewriting the EEPROM bin file) to 40c and notice that I can hold 40-43c on the second thermistor.

This should not adversely affect anyone, as 40c is significantly above ambient and can only make things better.

If you wish I can generate a pull request. I'm still on the 16/08/11 build r04fd5 and am hesitant to try this new Optiboot stuff.

A related issue could be just allowing the user to manually override the EEPROM setting via LCD.