Add 20/26MHz Flash frequencies for slow/cheap flash chips on the Generic ESP Board

[ChocolateFrogsNuts]

A number of non-genuine boards exist mainly from flea-bay sellers that
use under-sized and/or low quality flash chips which can not handle
a 40MHz FlashFreq properly.

This patch adds slower flash frequencies to the menu for generic ESP boards
so that these cheap knock-offs can be run in a stable manner, hopefully saving
some people a few headaches and keeping these boards out of landfill.

see #6366 final solution.

[Jason2866]

Imho bad cheap flash chips will not become reliable with tricks.
The should simply avoided. Strange errors will occur and Arduino core will be blamed
So the bin is the best place for this crap

[d-a-v]

@Jason2866 may be right, but arduino IDE may not prevent from experimenting with special values when using the generic board.
(imho the best solution is to test - also by this mean - a malfunctioning eeprom then replace it, and not throwing the esp8266ex chip away :)

[devyte]

@Jason2866 there's a reason that the generic board options allow selection of the flash frequency.

[ChocolateFrogsNuts]

@Jason2866 agreed that if the chip is in fact bad, it won't become good and it belongs in the bin.

Where this is useful is when the chip is storing the data correctly and reliably, but during the read cycle is not getting the data stable on the output fast enough for a 40mhz clock, resulting in intermittent bad reads.
The lower clock does in fact work in these cases - which I have seen with XM branded flash chips.
It is the difference between "why does this board crash randomly" and "this board is rock solid".

[Jason2866]

@d-a-v I replace flash chips (Puya...). But most user wont do ;-)

[ChocolateFrogsNuts]

@d-a-v I replace flash chips (Puya...). But most user wont do ;-)

haha, so do I when I have to, but on a $7 board that is being used to switch an output occasionally, which is better - buying a new flash chip + risking damage to the board, or just running it within it's limits? :-)
And yes, if it's actually mission/time-critical you buy the $15 board with the decent chip on it in the first place!