lib/oe/sstatesig: avoid reporting duplicate siginfo files from sstate

lib/oe/sstatesig: avoid reporting duplicate siginfo files from sstate

In find_siginfo(), which is used by bitbake-diffsigs among other things,
avoid adding a siginfo file from the sstate-cache where we've already
collected a sigdata file from the stamps directory with the same hash.
This avoids the possibility that the top two files (as picked by default
using the bitbake-diffsigs -t option) are for the same signature and
thus the tool would report no differences. In order to do that, just use
the hashfiles dict that we already have - we just need to change the
code to populate that even if we're collecting matching files without
looking for a fixed set of hashes (i.e. taskhashlist isn't set).

This replaces previous code in bitbake-diffsigs that attempted to filter
these out with limited success.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>