sys/psa_crypto: unify buffer size size handling for ed25519 backends #21821
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Contribution description
Ed25519 has fixed sizes for private keys, public keys, and signatures. Those can be checked on the algorithm dispatch layer once instead of duplicating code in the backends.
Additionally, we now consistently zero out key-material buffers on the stack in both the hardware and software backend.
Testing procedure
make -C tests/sys/psa_crypto_eddsa all flash teststill succeeds forBOARD=native(automatically usingc25519as software backend) and forBOARD=nrf52840dk(automatically usingdriver_cryptocell_310as HW backend).