Bump littlefs2-sys for littlefs 2.6

[sosthene-nitrokey]

This fixes a bug encountered in #33

According to the littlefs release notes, this shouldn't be a breaking change, but we should still test it nonetheless.

[elpiel]

I managed to compile and try to mount an SD card storage implementation with embedded-sdmmc.
Sadly on mount it gives me an error "Invalid".
I will try with a brand new filesystem just in case and try to format the card using littlefs-fuse once again.

Is there anything more specific to the migration process?
Do I need to update littlefs-fuse's version of littlefs?

[elpiel]

I manged to format and write to the SD card.
One of the issues that might have caused problems was littlefs-fuse. It also has releases for each littlefs version but releases haven't been updated properly.

Now with the master branch up to date I can also test the littlefs-fuse mounting and see if any problem arises from that.

[sosthene-nitrokey]

I did some testing once and it lead to the filesystem also failing to load though I didn't perform a thorough investigation as I was investigating another issue which I hoped would be solved by a more recent littlefs version, which wasn't the case.

I've added a couple test that generate a filesystem with the old version and reads them with the new and it seems to work fine.

[elpiel]

I've tested the new version by formatting the SD card using littlefs-fuse and then running some code on the device.
I still get an Invalid error.

I've been experiencing another issue when I format the SD card on the device. littlefs-fuse gives me error for incorrect block size but let me open an issue once I collect the details in coherent way.

[elpiel]

@sosthene-nitrokey any progress on upgrading littlefs?
There's a new release 2.7 from 2 weeks ago that adds some support for older littlefs protocol:

https://github.com/littlefs-project/littlefs/releases/tag/v2.7.0

[robin-nitrokey]

Replaced by #92.