Adapt ecdsa_verify_wrap() to new EC public key format

Previously, PSA used SubjectPublicKeyInfo structures to serialize EC public keys.
This has recently been changed to using ECPoint structures instead, but the wrapper
making PSA ECDSA verification available through Mbed TLS' PK API hasn't yet been
adapted accordingly - which is what this commit does.

Luckily, Mbed TLS' PK API offers two functions mbedtls_pk_write_pubkey()
and mbedtls_pk_write_pubkey_der(), the latter exporting a SubjectPublicKeyInfo
structure and the former exporting an ECPoint structure in case of EC public
keys. For the adaptation of the ECDSA wrapper ecdsa_verify_wrap() it is therefore
sufficient to use mbedtls_pk_write_pubkey() instead of mbedtls_pk_write_pubkey_der().

Author: Hanno Becker

library/pk_wrap.c

@@ -553,7 +553,7 @@ static int ecdsa_verify_wrap( void *ctx, mbedtls_md_type_t md_alg,
     int key_len;
     /* see ECP_PUB_DER_MAX_BYTES in pkwrite.c */
     unsigned char buf[30 + 2 * MBEDTLS_ECP_MAX_BYTES];
-    unsigned char *p = (unsigned char*) sig;
+    unsigned char *p;
     mbedtls_pk_info_t pk_info = mbedtls_eckey_info;
     psa_algorithm_t psa_sig_md, psa_md;
     psa_ecc_curve_t curve = mbedtls_psa_translate_ecc_group(
@@ -567,7 +567,8 @@ static int ecdsa_verify_wrap( void *ctx, mbedtls_md_type_t md_alg,
      * re-construct one to make it happy */
     key.pk_info = &pk_info;
     key.pk_ctx = ctx;
-    key_len = mbedtls_pk_write_pubkey_der( &key, buf, sizeof( buf ) );
+    p = buf + sizeof( buf );
+    key_len = mbedtls_pk_write_pubkey( &p, buf, &key );
     if( key_len <= 0 )
         return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
 
@@ -603,6 +604,7 @@ static int ecdsa_verify_wrap( void *ctx, mbedtls_md_type_t md_alg,
         goto cleanup;
     }
 
+    p = (unsigned char*) sig;
     if( ( ret = extract_ecdsa_sig( &p, sig + sig_len, buf,
                                    signature_part_size ) ) != 0 )
     {