FS: Add support for directories, block cache and journaling

[danodus]

The filenames are currently inside the FAT structure, so it don't scale well when we increase the number of files (1024 and beyond).
We would like the following structure instead:

Fat:

1. File infos
2. Blocks

File info:

1. First block table index (0xFFFF if entry not used)
2. Size (in bytes)

The on-disk structure is the following:

1. Magic identifier
2. Fat structure
3. Data

The first file info entry is reserved for the filenames. The index of the file info entry is the same as the index in the list of filenames.