Changed lfs_dir_read to return 0 on success

Before:
If an entry is found, lfs_dir_read returns 1
If at end of directory, lfs_dir_read returns 0

After:
If an entry is found, lfs_dir_read returns 0
If at end of directory, lfs_dir_read returns LFS_ERR_NOENT

---

lfs_dir_read is a bit of an odd function. It's supposed to match
lfs_file_read, but instead of reading bytes, it reads directory
entries.

Its POSIX equivalent, readdir, indicates whether or not we reached
the end of the directory by returning a NULL pointer:

struct dirent *readdir(DIR *dirp);

But this API needed to change for littlefs, since we don't use errno,
instead returning errors as signed integers through all API functions,
including lfs_dir_read.  So, in an effort to match lfs_file_read,
lfs_dir_read returned the "number" of directory entries read, limited
to either 0 or 1.

Perhaps unsurprisingly, this turned out to be confusing to users,
and a better API was proposed hathach to instead return either 0
or LFS_ERR_NOENT, an error which can't occur for other reasons in
lfs_dir_read.

Suggested by hathach

lfs.h

@@ -619,8 +619,9 @@ int lfs_dir_close(lfs_t *lfs, lfs_dir_t *dir);
 // Read an entry in the directory
 //
 // Fills out the info structure, based on the specified file or directory.
-// Returns a positive value on success, 0 at the end of directory,
-// or a negative error code on failure.
+//
+// Returns 0 on success, LFS_ERR_NOENT if there are no more directry entries,
+// or another negative error code on failure.
 int lfs_dir_read(lfs_t *lfs, lfs_dir_t *dir, struct lfs_info *info);
 
 // Change the position of the directory