SingleFileDisk allows for exporting a file from the onboard LittleFS
filesystem to a PC through an emulated FAT drive when connected. The
PC can open and copy the file, as well as delete it, but the PC has no
access to the main onboard LittleFS and no actual on-flash FAT
structures are used.
This is handy for things like data loggers. They can run connected to
USB power for some time, and then connected to a PC to dowmload the CSV
log recorded.
It's almost 2023, allow LFN (long file names) on the emulated USB disk.
Reduce the disk buffer size to 64 bytes. The buffer is statically
allocated so it's always present, even in non-USB disk mode, meaning
all apps will pay the RAM price for it. 64 bytes is slower to read
but works and saves ~1/2KB of heap for all apps.
