Remove custom COSE header decoding code

[letmaik]

The t_cose 1.x release does not support decoding of custom header parameters. This functionality however is part of the dev branch of t_cose which will become 2.x.

As we need custom parameters, we currently decode the header ourselves in app/src/cose.h. Once t_cose 2.x or a pre-release of it is out, we can upgrade and remove the custom code.

[plietar]

As a side note, now that CCF vendors t_cose, if we update to CCF 3.x we'll be restricted to whatever version they are using. They might object to using a pre-release version.

(In fact I tried to build with CCF 3.x and it failed because the t_cose version they use doesn't have the EdDSA support, I need to get them to update).

[achamayou]

CCF will object to using pre-releases. But it also has some code to support custom header parameters it'd rather do without, and will therefore gladly upgrade to t_cose 2.x releases, as soon as they become available.