If you run bitbake-diffsigs against two differing sigdata files from
nostamp tasks it shows no difference despite the differing checksum.
Change the code so this shows up as a nostamp 'taint' and at least
makes the issue clearer to the end user.
(Bitbake rev: 97679d18955dadaa34f9450564e44da99984d140)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>