feat(fatfs): Implement global FLASH lock

- 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