We recalculate the taskhash to ensure the version we have matches
what we think it should be. When we write out a sigdata file, use
the calculated value so that we don't overwrite any existing file.
This leaves any original taskhash sigdata file intact to allow a
debugging comparison.
(Bitbake rev: 291353b711670ce2da3d45617fc96520bdf09d3f)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>