sstate.bbclass: Only sign packages at the time of their creation

sstate.bbclass: Only sign packages at the time of their creation

The purpose of the change is to never sign a package not created by
the build itself.

sstate_create_package is refactored into Python and re-designed
to handle signing inside the function. Thus, the signing should never apply
to existing sstate packages. The function is therefore renamed into
sstate_create_and_sign_package.
The creation of the archive remains in a separate shellscript function.

Co-authored-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Tobias Hagelborn <tobias.hagelborn@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>