Small fatfs usability fixes

[chrysn]

Contribution description

This is a collection of the small things that make life easier for someone who wants to give fatfs a try:

• Name the tarball of tests/pkg_fatfs .tar.bz2 (tools like atool do look at extensions)
• Make the extraction routine work even when bin/ has not yet been created (with Makefile style fixes as bycatch)
• Tell user what to do when enabling fatfs to get a usable example (because the disk can't format itself)

Testing procedure

• In tests/pkg_fatfs, run (there is not automation for this, is there?):

  $ make image all term
  > mount 0
  mounting file system image...
  [OK]
  Volume name: 
  0,124 GiB of 0,124 GiB available
  > ls
  //TEST.TXT
  

• In examples/filesystem, follow the new set of instructions and observe that indeed the file system becomes usable with a TEST.TXT present from startup.

[benpicco]

We can also wire up format, the only issue is how to provide memory for the workbuffer #14430

[chrysn]

Yeah, but that has dragged on long, and it's not like getting a ready-made file system with content rather than a blank one (or seeing that disk geometries matter here) is not helping the user.

(Does that PR need a review to get unstuck, or discussion, or changes?)

[benpicco]

I think it needs consensus on where to get the 512 bytes memory needed while formatting:

• dynamic allocation with malloc() and free()
• static allocation
• put it on the stack and hope for the best

[benpicco]

regardless, those doc improvements are still useful.