Merge "Document expectations of DICE mode"

of a DKCertChain in AdditionalDKSignatures (see

[CertificateRequest](#certificaterequest)).

#### Mode

The Open Profile for DICE specifies four possible modes with the most important

mode being `normal`. A certificate must only set the mode to `normal` when all

of the following conditions are met when loading and verifying the software

component that is being described by the certificate:

*   verified boot with anti-rollback protection is enabled

*   only the verified boot authorities for production images are enabled

*   debug ports, fuses or other debug facilities are disabled

*   device booted software from the normal primary source e.g. internal flash

If any of these conditions are not met then it is recommended to explicitly

acknowledge this fact by using the `debug` mode. The mode should never be `not

configured`.

#### Configuration descriptor

The Open Profile for DICE allows for an arbitrary configuration descriptor. For

BCC entries, this configuration descriptor is a CBOR map with the following

optional fields. If no fields are relevant, an empty map should be encoded.