Support persistent writes to flash image

[msn444]

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Wokwi doesn't seem to support flash writes that persist to future sessions. For example, a write to a SPIFFS partition doesn't affect the .bin file and therefore has no effect on future sessions. User changes to device settings are lost between sessions.

More generally, I'd like all flash writes to persist between sessions, including to SPIFFS, NVS, OTA partitions, and the OTA data partition. These are all aspects of the system that would be really nice to be able to develop and test in a simulator.

Ideally it would work like any other VM, where the mass storage device is emulated from an image file on the host and can be modified by the VM.

Originally posted by @msn444 in https://github.com/wokwi/wokwi-features/issues/431#issuecomment-1891396892

[tyeth]

Maybe a cleanStart boolean flag set to false in the diagram can stipulate that users want persistence, otherwise normal behaviour assumed. This way no functionality breaks, and the default is no extra storage cost.