diskio.c

- implement de-initialization
- wrap filesystem operations in semaphore

- Introduce EpicFileSystem object and move epic_file_FOO(...) imlementations into
  efs_FOO(EpicFileSystem*, ...) functions.
- epic_file_FOO(...) functions are now wrappers around the _fs_ functions, but lock and unlock the global filesystem object before&after calls.
  This way, all efs_ functions can assume that the necessary lock has been acquired.
- libff: don't use FF_FS_REENTRANT, our own FS lock is enough