pkg/fatfs: add vfs integration

Makefile.dep

core/include/kernel_defines.h

cpu/native/mtd/mtd_native.c

pkg/fatfs/Makefile.include

pkg/fatfs/fatfs_vfs/Makefile

pkg/fatfs/fatfs_vfs/fatfs_vfs.c

sys/include/fs/fatfs.h

sys/include/vfs.h

tests/pkg_fatfs/create_fat_image_file.sh

tests/pkg_fatfs/riot_fatfs_disk.tar.gz

tests/pkg_fatfs_vfs/Makefile

tests/pkg_fatfs_vfs/create_fat_image_file.sh

tests/pkg_fatfs_vfs/main.c

tests/pkg_fatfs_vfs/riot_fatfs_disk.tar.gz

tests/pkg_fatfs_vfs/tests/01-run.py