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/entropy.h b/include/polarssl/entropy.h
index 039f5cd..539e5cb 100644
--- a/include/polarssl/entropy.h
+++ b/include/polarssl/entropy.h
@@ -81,9 +81,9 @@
 /**
  * \brief           Entropy context structure
  */
-typedef struct 
+typedef struct
 {
-    sha4_context    accumulator;
+    sha512_context  accumulator;
     int             source_count;
     source_state    source[ENTROPY_MAX_SOURCES];
 #if defined(POLARSSL_HAVEGE_C)