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_srv.c b/library/ssl_srv.c
index 0c18f0d..2bf3725 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -149,7 +149,7 @@
             p += 2;
             continue;
         }
-#if defined(POLARSSL_SHA4_C)
+#if defined(POLARSSL_SHA512_C)
         if( p[0] == SSL_HASH_SHA512 )
         {
             ssl->handshake->sig_alg = SSL_HASH_SHA512;
@@ -161,7 +161,7 @@
             break;
         }
 #endif
-#if defined(POLARSSL_SHA2_C)
+#if defined(POLARSSL_SHA256_C)
         if( p[0] == SSL_HASH_SHA256 )
         {
             ssl->handshake->sig_alg = SSL_HASH_SHA256;
@@ -1380,7 +1380,7 @@
                     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;
@@ -1388,7 +1388,7 @@
                     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;