ArduinoISP: Add an option to enable a 4 MHz clock signal on pin 3

[per1234]

Moved from arduino/Arduino#8757 by @procsynth

This clock signal can be used to reprogram the fuses of an AVR that
has been set wrong (ie. when an external clock has been configured but
the board/chip does not have one).

This is disabled by default. We check that we compile for known chips
before enabling the clock generation. Maybe there is other chips that
support this code.

[per1234]

From facchinm on 2019-04-18

Hi @procsynth ,
thanks for your PR.

ArduinoISP sketch is meant to run on all Arduinos (also the ones with ARM chips), so it would be nice to keep the sketch functionalities in sync between all the targets.

The functionality is interesting and should be expanded a bit, like having a couple of options for the pin and the frequency to use, and less magic numbers 😉

[per1234]

@facchinm requested changes on 2019-04-18

ArduinoISP sketch is meant to run on all Arduinos (also the ones with ARM chips), so it would be nice to keep the sketch functionalities in sync between all the targets.

[per1234]

From @procsynth on 2019-04-23

Hi @facchinm, thanks for your reply. I did not think in the scope of the whole Arduino project.
I'll make the changes requested, explain and test the code a little better when I have some time :)

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