bitbake: siggen: Ensure taskhash mismatches don't override existing data

bitbake: siggen: Ensure taskhash mismatches don't override existing data

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: dac68af6f4add9c99cb7adcf23b2ae89b96ca075)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>