None is a bad choice of return value for functions used in variables
(strings) as a failure results in concatination errors. Use a string
with a clear meaning that can be searched for instead.
(From OE-Core rev: c04f04e714ede5d3904058ec82459139ed5e42fa)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>