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/include/polarssl/ssl.h b/include/polarssl/ssl.h
index e0e4bd8..01d8839 100644
--- a/include/polarssl/ssl.h
+++ b/include/polarssl/ssl.h
@@ -382,10 +382,10 @@
     /*
      * Checksum contexts
      */
-     md5_context fin_md5;
-    sha1_context fin_sha1;
-    sha2_context fin_sha2;
-    sha4_context fin_sha4;
+       md5_context fin_md5;
+      sha1_context fin_sha1;
+    sha256_context fin_sha256;
+    sha512_context fin_sha512;
 
     void (*update_checksum)(ssl_context *, const unsigned char *, size_t);
     void (*calc_verify)(ssl_context *, unsigned char *);