Build failure with PSA and RSA but not PK_PARSE

[gilles-peskine-arm]

When MBEDTLS_PSA_CRYPTO_C and MBEDTLS_RSA_C are enabled but not MBEDTLS_PK_PARSE_C, the build fails, because PSA relies on pk_parse functions to load RSA keys.

library/psa_crypto_rsa.c: In function ‘mbedtls_psa_rsa_load_representation’:
library/psa_crypto_rsa.c:85:13: error: implicit declaration of function mbedtls_pk_parse_key’; did you mean ‘mbedtls_pk_free’? [-Werror=implicit-function-declaration]
   85 |             mbedtls_pk_parse_key( &ctx, data, data_length, NULL, 0,
      |             ^~~~~~~~~~~~~~~~~~~~
      |             mbedtls_pk_free
library/psa_crypto_rsa.c:89:13: error: implicit declaration of function mbedtls_pk_parse_public_key’; did you mean ‘mbedtls_pk_write_pubkey’? [-Werror=implicit-function-declaration]
   89 |             mbedtls_pk_parse_public_key( &ctx, data, data_length ) );
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
      |             mbedtls_pk_write_pubkey

This applies to both 2.28 and development.

This hasn't worked since at least 2.24 when we changed the internal representation of RSA keys (and even before that I think it would only have worked for internally generated volatile keys, with runtime errors for key import or persistent keys).

The quick fix is to add a requirement in check_config.h. In development, we might do what we're already partially doing (though not quite correctly) for MBEDTLS_PK_WRITE_C, which is to force it on when required in build_info.h.