env: Allow returning errors from hdelete_r()

At present this function returns 1 on success and 0 on failure. But in
the latter case it provides no indication of what went wrong.

If an attempt is made to delete a non-existent variable, the caller may
want to ignore this error. This happens when setting a non-existent
variable to "", for example.

Update the function to return 0 on success and a useful error code on
failure. Add a function comment too.

Make sure that env_set() does not return an error if it is deleting a
variable that doesn't exist. We could update env_set() to return useful
error numbers also, but that is beyond the scope of this change.

Signed-off-by: Simon Glass <sjg@chromium.org>

wip

test/env/hashtable.c

@@ -80,7 +80,7 @@ static int htab_create_delete(struct unit_test_state *uts,
 		ut_asserteq_str(key, ritem->key);
 		ut_asserteq_str(key, ritem->data);
 
-		ut_asserteq(1, hdelete_r(key, htab, 0));
+		ut_asserteq(0, hdelete_r(key, htab, 0));
 	}
 
 	return 0;