Integrate meta-seco-edgehog/kirkstone/develop_sign_key

Commit: https://git.seco.com/yocto_ng/layers/seco/meta-seco-edgehog/-/commit/c7b69387b653f55d4755e9c691659e29cda741eb

[rauc][certs] Rework of the custom keyring

The custom keyring is now directly included via the extra search path. In this step, the example keyring was also moved to a new insecure-keys folder. Also see commit 4831fa468f57f377b96c0c48ea3f587657009fe8.