fs: Set MAX_FILE_NAME appropiately with LFN and FATFS

Try to define MAX_FILE_NAME to the appropriate max length based on the
selected filesystm. Otherwise fallback to a standard filename of 12
(made up of 8.3 : <filename>.<extension>)

Signed-off-by: Roman Vaughan <nzsmartie@gmail.com>
Signed-off-by: David Brown <david.brown@linaro.org>