Integrate meta-seco-edgehog/kirkstone/develop_sign_key

Commit: yocto_ng/layers/seco/meta-seco-edgehog@4831fa46

[rauc][cert] Moved the example cert/key to the insecure-keys folder

Moved the example cert/key to a new folder called insecure-keys to further emphasise that they must not be used in production environments.

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Commit: yocto_ng/layers/seco/meta-seco-edgehog@ea6052de

[rauc][cert] Enabled install of custom keyring for RAUC

If a user creates custom RAUC bundles, we also need to include the cert as keyring into the image. The keyring file is used to verify future releases when updating via OTA.

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Commit: yocto_ng/layers/seco/meta-seco-edgehog@71bd4a5c

[rauc][signing] Enabled custom RAUC key/cert

The user can now set a custom key and cert for the signing of RAUC bundles.