signing-keys: Make signing keys the only publisher of keys

signing-keys: Make signing keys the only publisher of keys

Previously the keys were put into the os-release package. The package
indexing code was also deploying the keys rather than only using the keys.

This change makes signing-keys.bb the only publisher of the keys and also
uses standard tasks that already have sstate.

Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>