Added support for 40MHz crystal frequency

[jeremypoulter]

This is needed for some ESP8266 modules that, for example the module in this SmartPlug, that have a 40Mhz crystal rather than the 'normal' 26MHz.

[torntrousers]

Hi Jeremy, that SmartPlug looks interesting but I can't find much on it, do you have any links to more about it - photos of inside or info about re-programing it etc?

[jeremypoulter]

Some info on the module can be found here; https://detail.1688.com/offer/45107082796.html and there is some discussion on it in this forum; https://www.letscontrolit.com/forum/viewtopic.php?t=2159 however I am still working on the full documentation of my efforts with this plug.

[codecov-io]

Codecov Report

Merging #3025 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3025   +/-   ##
=======================================
  Coverage   27.82%   27.82%           
=======================================
  Files          20       20           
  Lines        3626     3626           
  Branches      656      656           
=======================================
  Hits         1009     1009           
  Misses       2441     2441           
  Partials      176      176

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[gvJaime]

I've had issues #3004 and #3094 with the Arduino UNO WiFi, because of this little 40MHz crystal detail.

It would be great that this pull request gets merged because right now I have to modify the framework by hand to make it work. It would be easier with this simple flag.

[jeremypoulter]

Somewhat frustratingly this change was added (not quite as cleanly IMHO) by #3121 so closing this