Finish the PSA subsystem initialization draft

[gilles-peskine-arm]

Finish the work in progress on a PSA subsystem interface (psa_crypto_init_subsystem).

Mbed TLS needs this because some use cases need partial initialization, e.g. to verify a signature in boot code before a TRNG is available.

Private link to the spec draft: https://github.com/ARM-software/psa-crypto-api/pull/536. This task must be done by someone from Arm. I'm filing it here because it's likely to be someone from the Mbed TLS team.

Definition of done: approved by the PSA Crypto API working group, not implemented in Mbed TLS yet.

[gilles-peskine-arm]

We have not done this in time for 1.0. At this point it doesn't look like it'll be in the 1.1 LTS branch either. It's still on the backlog but with no scheduled date.

[gilles-peskine-arm]

Since Andrew is now doing the specification work, this issue is no longer relevant. The issue against the PSA API is public nowadays: ARM-software/psa-api#202

The implementation work is covered by Mbed-TLS/TF-PSA-Crypto#606 and its follow-ups.