classes/buildhistory: save output file signatures for sstate tasks

Save a file per task listing sha256sums for each file staged, i.e.
the output of the task. Some caveats:

1) This only covers sstate tasks since it uses SSTATEPOSTUNPACKFUNCS,
   however those are generally the most interesting in terms of output
   anyway.
2) The signature is taken before applying any relocations, so any
   relocated files will actually have different signatures, but that's
   churn that you probably won't want to see here.
3) At the moment if you run the same build twice without sstate you will
   very likely see changes in the output for certain tasks due to things
   like timestamps being present in the binary output. Fixing that is
   a general Linux ecosystem problem - see this page for our efforts to
   resolve it on our side:
     https://wiki.yoctoproject.org/wiki/Reproducible_Builds

NOTE: you need to set your BUILDHISTORY_FEATURES value to include
"task" to enable collection of these signatures as it is is disabled by
default.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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