meta/lib/oe/sstatesig: fix locating stamp files

meta/lib/oe/sstatesig: fix locating stamp files

Fixes "bitbake-diffsigs -t" for changes to the stamp directory layout,
and this time uses the actual value of STAMP to get the location of
sigdata files in the stamp directory rather than trying to do it
manually, which should be a little more robust.

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