SHA2 renamed to SHA256, SHA4 renamed to SHA512 and functions accordingly

The SHA4 name was not clear with regards to the new SHA-3 standard. So
SHA2 and SHA4 have been renamed to better represent what they are:
SHA256 and SHA512 modules.
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 96fba23..93d81a6 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -130,13 +130,13 @@
     /*
      * Prepare signature_algorithms extension (TLS 1.2)
      */
-#if defined(POLARSSL_SHA4_C)
+#if defined(POLARSSL_SHA512_C)
     sig_alg_list[sig_alg_len++] = SSL_HASH_SHA512;
     sig_alg_list[sig_alg_len++] = SSL_SIG_RSA;
     sig_alg_list[sig_alg_len++] = SSL_HASH_SHA384;
     sig_alg_list[sig_alg_len++] = SSL_SIG_RSA;
 #endif
-#if defined(POLARSSL_SHA2_C)
+#if defined(POLARSSL_SHA256_C)
     sig_alg_list[sig_alg_len++] = SSL_HASH_SHA256;
     sig_alg_list[sig_alg_len++] = SSL_SIG_RSA;
     sig_alg_list[sig_alg_len++] = SSL_HASH_SHA224;
@@ -892,7 +892,7 @@
             *md_alg = POLARSSL_MD_SHA1;
             break;
 #endif
-#if defined(POLARSSL_SHA2_C)
+#if defined(POLARSSL_SHA256_C)
         case SSL_HASH_SHA224:
             *md_alg = POLARSSL_MD_SHA224;
             break;
@@ -900,7 +900,7 @@
             *md_alg = POLARSSL_MD_SHA256;
             break;
 #endif
-#if defined(POLARSSL_SHA4_C)
+#if defined(POLARSSL_SHA512_C)
         case SSL_HASH_SHA384:
             *md_alg = POLARSSL_MD_SHA384;
             break;