Add output size parameter to signature functions

The functions mbedtls_pk_sign(), mbedtls_pk_sign_restartable(),
mbedtls_ecdsa_write_signature() and mbedtls_ecdsa_write_signature_restartable()
now take an extra parameter indicating the size of the output buffer for the
signature.

No change to RSA because for RSA, the output size is trivial to calculate.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/x509write_crt.c

@@ -474,7 +474,7 @@ int mbedtls_x509write_crt_der( mbedtls_x509write_cert *ctx,
     }
 
     if( ( ret = mbedtls_pk_sign( ctx->issuer_key, ctx->md_alg,
-                                 hash, 0, sig, &sig_len,
+                                 hash, 0, sig, sizeof( sig ), &sig_len,
                                  f_rng, p_rng ) ) != 0 )
     {
         return( ret );