packaged-staging: Grab the staging lock before we copy and test the stamps so

packaged-staging: Grab the staging lock before we copy and test the stamps so
that we don't have other threads try and validate or install based on them
being valid.  This is a potential issue for certain BB_STAMP_POLICY (such as
the default) values and enough concurrent threads.

Signed-off-by: Tom Rini <tom_rini@mentor.com>