Break down mbedtls/psa_util.h

[gilles-peskine-arm]

The header mbedtls/psa_util.h contains miscellaneous functions that tie the PSA API with the legacy crypto API. It no longer makes sense when the legacy API is not the primary API.

The goal of this issue is to remove include/mbedtls/psa_util.h and move its functions to other headers:

• mbedtls_psa_get_random, MBEDTLS_PSA_RANDOM_STATE: only useful to assist other legacy APIs. Move to psa_util_internal.h.
• mbedtls_ecc_group_to_psa, mbedtls_ecc_group_from_psa: only useful in combination with ecp.h and other interfaces that use a legacy group ID. Move to ecp.h (which is becoming internal).
• mbedtls_md_psa_alg_from_type, mbedtls_md_type_from_psa_alg: either move to md.h or remove altogether, depending on the fate of mbedtls_md_type_t in the md.h redesign.
• mbedtls_ecdsa_raw_to_der, mbedtls_ecdsa_der_to_raw: these functions are in fact not legacy-PSA bridges, but PSA helpers, and remain useful for PSA API users. Move to asn1write.h and asn1.h respectively.

Functions that become internal may become obsolete with the work towards TF-PSA-Crypto 1.0. Removing the functions is out of scope here (but may be done at once if the functions are no longer used by the time we remove psa_util.h).