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.c

@@ -2160,18 +2160,19 @@ static int lfs_dir_rawread(lfs_t *lfs, lfs_dir_t *dir, struct lfs_info *info) {
         info->type = LFS_TYPE_DIR;
         strcpy(info->name, ".");
         dir->pos += 1;
-        return true;
+        return 0;
     } else if (dir->pos == 1) {
         info->type = LFS_TYPE_DIR;
         strcpy(info->name, "..");
         dir->pos += 1;
-        return true;
+        return 0;
     }
 
     while (true) {
         if (dir->id == dir->m.count) {
             if (!dir->m.split) {
-                return false;
+                // reached end of traversal
+                return LFS_ERR_NOENT;
             }
 
             int err = lfs_dir_fetch(lfs, &dir->m, dir->m.tail);
@@ -2194,7 +2195,7 @@ static int lfs_dir_rawread(lfs_t *lfs, lfs_dir_t *dir, struct lfs_info *info) {
     }
 
     dir->pos += 1;
-    return true;
+    return 0;
 }
 
 static int lfs_dir_rawseek(lfs_t *lfs, lfs_dir_t *dir, lfs_off_t off) {