Add LittleFS support for internal flash storage

[ThomasFarstrike]

Overview

This PR adds optional LittleFS support to retro-go's internal flash storage subsystem, allowing retro-go to use LittleFS as an alternative to FAT for managing internal flash partitions.

Motivation

During development of the fri3d-2024 target, we encountered unexplained sporadic incompatibility issues between retro-go and MicroPython when both were sharing the same internal storage partition. These issues appeared to stem from differences in filesystem implementations—potentially due to different FAT versions, wear-leveling layer configurations, or other compatibility settings.

Since MicroPython defaults to LittleFS for internal storage (the modern standard for embedded systems), the most pragmatic solution is to add LittleFS support to retro-go. This way, we achieve good interoperability when both systems need to access the same internal storage.

Why LittleFS?

LittleFS is an excellent fit for microcontroller applications:

• Wear-leveling: Distributes writes evenly across flash memory, extending device lifespan
• Power-loss resilience: Designed to handle unexpected power failures without corruption
• Smaller footprint: Minimal RAM and code size requirements, ideal for embedded systems
• Modern standard: Increasingly adopted across embedded platforms (MicroPython, Zephyr, etc.)
• Compatibility: Aligns retro-go with the ecosystem's direction

Changes

• Added optional RG_STORAGE_FLASH_PARTITION_LITTLEFS configuration flag
• When enabled, internal flash partitions are mounted as LittleFS instead of FAT
• Maintains backward compatibility—FAT remains the default when the flag is not set
• Updated storage initialization and cleanup routines to handle both filesystems

Testing

This implementation has been tested with:

• retro-core: Verified core emulation functionality with LittleFS storage
• prboom-go: Confirmed game loading and save state management work correctly
• fri3d-2024 target: Validated interoperability with MicroPython on shared storage

Backward Compatibility

The changes are fully backward compatible. Existing configurations continue to use FAT by default. LittleFS support is opt-in via the RG_STORAGE_FLASH_PARTITION_LITTLEFS configuration flag.

[ThomasFarstrike]

I included esp_littlefs as a git submodule to avoid copy-paste. But I'm not sure whether you'd prefer a git submodule or a source copy instead. I'm starting to suspect the latter, otherwise you have to clone with "git clone --recurse-submodules".

Let me know if you want me to change it :-)

[ThomasFarstrike]

Or rg_tool.py doing the clone?

[ThomasFarstrike]

I tested this with esp-idf 5.4 , which worked fine except for a few deprecation warnings.
Do you think it's time to update BUILDING.md to make it official?

[ducalex]

I think I would prefer to vendor in esp_littlefs (source copy) instead of using a submodule! I'm reluctant to add external remote dependencies to retro-go for multiple reasons, and that one is small and self-contained so it should be fine to copy it.

[ThomasFarstrike]

Sure, that makes sense. Done!

I also added a little README.md in the components folder to explain what is there; especially the exact versions of the source copies.

[ducalex]

A bit of a tangent but do you know why esp_littlefs doesn't implement access_p?

I imagine that littlefs has no concept of permissions but it could always return success on R_OK|W_OK|X_OK and only concern itself with F_OK? Am I missing something? That would be more elegant than having to patch applications, which I really want to avoid when possible.

[ThomasFarstrike]

A bit of a tangent but do you know why esp_littlefs doesn't implement access_p?

Hmmm, no I don't know why that is...