Added deletion to custom attributes

This follows from enabling tag deletion, however does require some
consideration with the APIs.

Now we can remove custom attributes, as well as determine if an attribute
exists or not.

lfs.h

@@ -82,6 +82,7 @@ enum lfs_error {
     LFS_ERR_INVAL       = -22,  // Invalid parameter
     LFS_ERR_NOSPC       = -28,  // No space left on device
     LFS_ERR_NOMEM       = -12,  // No more memory available
+    LFS_ERR_NOATTR      = -61,  // No data/attr available
     LFS_ERR_NAMETOOLONG = -36,  // File name too long
 };
 
@@ -456,7 +457,8 @@ int lfs_stat(lfs_t *lfs, const char *path, struct lfs_info *info);
 // Custom attributes are uniquely identified by an 8-bit type and limited
 // to LFS_ATTR_MAX bytes. When read, if the stored attribute is smaller than
 // the buffer, it will be padded with zeros. If the stored attribute is larger,
-// then it will be silently truncated.
+// then it will be silently truncated. If no attribute is found, the error
+// LFS_ERR_NOATTR is returned and the buffer is filled with zeros.
 //
 // Returns the size of the attribute, or a negative error code on failure.
 // Note, the returned size is the size of the attribute on disk, irrespective
@@ -469,13 +471,19 @@ lfs_ssize_t lfs_getattr(lfs_t *lfs, const char *path,
 //
 // Custom attributes are uniquely identified by an 8-bit type and limited
 // to LFS_ATTR_MAX bytes. If an attribute is not found, it will be
-// implicitly created, and setting the size of an attribute to zero deletes
-// the attribute.
+// implicitly created.
 //
 // Returns a negative error code on failure.
 int lfs_setattr(lfs_t *lfs, const char *path,
         uint8_t type, const void *buffer, lfs_size_t size);
 
+// Removes a custom attribute
+//
+// If an attribute is not found, nothing happens.
+//
+// Returns a negative error code on failure.
+int lfs_removeattr(lfs_t *lfs, const char *path, uint8_t type);
+
 
 /// File operations ///