No sub dirs created

[TimonPeng]

Thank you for your contribution to the community! Looks like platformio has been integrated with this project and the mkfatfs binary will be called to create the fatfs image file when set board_build.filesystem = fatfs.

But seems doesn't handles sub-directories:

dir_RAM_fs: /fatfs/assets  dir_pc: data/assets
Sub dir: index.js  full dir_full_path_s: data/assets/index.js   full dir_full_path_d: /fatfs/assets/index.js
error: failed to open "/fatfs/assets" for writing