samples/fs/fat_fs: Allow creating entries for demonstartion

The commits adds Kconfig option CONFIG_SAMPLE_FATFS_CREATE_SOME_ENTRIES
which causes the sample to attempt to create one file and one
directory, in case if listing does not show anything, just
to demnonstrate how FAT FS operations work on internal flash.

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