Add option to disable long file name (LFN) support

[cowellmi]

By setting FF_USE_LFN to 0 we can save ~60KB of flash memory.

Disabling LFN restricts the user to using 8.3 filenames.

I propose adding a no_lfn build tag to avoid breaking changes.

❯ tinygo build -o /dev/null -target=feather-m0 -size=short ./examples/console/fatfs/sdcard/
   code    data     bss |   flash     ram
 160976    2348    5452 |  163324    7800

❯ tinygo build -o /dev/null -target=feather-m0 -tags=no_lfn -size=short ./examples/console/fatfs/sdcard/
   code    data     bss |   flash     ram
  98832    2348    5452 |  101180    7800