fs/fatfs: Move FF_USE_LFN logic to Kconfig

Commit add hidden Kconfig option CONFIG_FS_FATFS_FF_USE_LFN
that is passed to ELM FAT to define FF_USE_LFN configuration.
The FF_USE_LFN still depends, indirectly, on choice
CONFIG_FS_FATFS_LFN_MODE config options
CONFIG_FS_FATFS_LFN_MODE_BSS, FS_FATFS_LFN_MODE_STACK and
FS_FATFS_LFN_MODE_HEAP, but the logic has been moved out of
zephyr_fatfs_config.h into Kconfig file.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>