Allow hardcoding single signature hash at compile-time

This commit introduces the option MBEDTLS_SSL_CONF_SINGLE_HASH
which can be used to register a single supported signature hash
algorithm at compile time. It replaces the runtime configuration
API mbedtls_ssl_conf_sig_hashes() which allows to register a _list_
of supported signature hash algorithms.

In contrast to other options used to hardcode configuration options,
MBEDTLS_SSL_CONF_SINGLE_HASH isn't a numeric option, but instead it's
only relevant if it's defined or not. To actually set the single
supported hash algorithm that should be supported, numeric options

MBEDTLS_SSL_CONF_SINGLE_HASH_TLS_ID
MBEDTLS_SSL_CONF_SINGLE_HASH_MD_ID

must both be defined and provide the TLS ID and the Mbed TLS internal
ID and the chosen hash algorithm, respectively.
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index 87454b2..bb84207 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -569,6 +569,7 @@
     return( 0 );
 }
 
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_HASH)
 static int ssl_sig_hashes_for_test[] = {
 #if defined(MBEDTLS_SHA512_C)
     MBEDTLS_MD_SHA512,
@@ -584,6 +585,7 @@
 #endif
     MBEDTLS_MD_NONE
 };
+#endif /* !MBEDTLS_SSL_CONF_SINGLE_HASH */
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 
 /*
@@ -1693,7 +1695,9 @@
     {
         crt_profile_for_test.allowed_mds |= MBEDTLS_X509_ID_FLAG( MBEDTLS_MD_SHA1 );
         mbedtls_ssl_conf_cert_profile( &conf, &crt_profile_for_test );
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_HASH)
         mbedtls_ssl_conf_sig_hashes( &conf, ssl_sig_hashes_for_test );
+#endif
     }
 
     mbedtls_ssl_conf_verify( &conf, my_verify, NULL );