modules/fatfst: Add support for default ffconf.h FF_VOLUME_STRS override

[tbursztyka]

FatFS is pretty unflexible when it comes to volume names where all are hardcoded and fixed.

So let's patch Zephyr to expose a Kconfig based volumes string which will be parsed by fatfs to fill up it's volumes list.

see fatfs's related PR zephyrproject-rtos/fatfs#17

[Laczen]

@tbursztyka, my knowledge on the fatfs system is to limited to evaluate if the proposal is the correct approach. Removing the review request.

[de-nordic]

I am not in favour of extensive modifications to FAT FS code base, when they can be avoided.
I would rather have entire get_ldnumber function #ifdefed, which is two line modification and have it provided by module additions within Zephyr. This way we can even provide alternative path support where ':' is not required, which would make LittleFS and FAT FS use same path format and make it easier to switch between them.

[github-actions]

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