meta/lib/oe/sstatesig: fix finding siginfo across sstate and stamps

meta/lib/oe/sstatesig: fix finding siginfo across sstate and stamps

Enable comparisons with bitbake-diffsigs -t between a task whose output
exists in sstate and a recently executed version of the same task, as
well as produce the correct error message if only one signature is found
in the stamps directory (not enough for a comparison but enough to tell
the user that they at least got the task/recipe name correct.)

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