classes/sstate: don't use unsigned sstate when verification enabled

classes/sstate: don't use unsigned sstate when verification enabled

When signature verification of shared state objects is enabled
(SSTATE_VERIFY_SIG) use of an unsigned object, even though it produces a
warning, seems unexpected. Instead skip unsigned objects and force the
non-accelerated task to be run.

Signed-off-by: Joshua Lock <jlock@vmware.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>