Add integrity checks to file system

[PatrickKa]

Description

I think littlefs already supports some kind of integrity check, but I am not sure. We have to check out its documentation. If it does not support it already, we add a CRC32 checksum at the end of all files.

[PatrickKa]

Blocked by #186

[PatrickKa]

Littlefs does check if writes were successful by immediately reading back each written page, but it does not detect data corruption after a finished write. We can, however, add checksums right in Program() and Read() which we must provide to littlefs to know how to write to a device (in our case flash or RAM). This has the advantage that the user of the file system does not know or care about the checksums. Also, since we have one checksum per page (256 bytes) instead of per file, we get better error detection for files > 256 B. Finally, this also means that this issue is no longer blocked by #186. It is actually part of the error handling now.