The parameters of the MTD device (`sector_count`, `page_size`)
are only known after the device has been initialized.
Init the device before using it to avoid reading all 0 on e.g. SD cards.
Two new fileds added in spiffs descriptor:
- base_addr: base address of the partition
- part_block_count: number of sectors in the partition
If part_block_count is 0, the whole underlying MTD is used