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/x509parse.c b/library/x509parse.c
index c49c48a..6e37a85 100644
--- a/library/x509parse.c
+++ b/library/x509parse.c
@@ -55,10 +55,10 @@
 #if defined(POLARSSL_SHA1_C)
 #include "polarssl/sha1.h"
 #endif
-#if defined(POLARSSL_SHA2_C)
+#if defined(POLARSSL_SHA256_C)
 #include "polarssl/sha2.h"
 #endif
-#if defined(POLARSSL_SHA4_C)
+#if defined(POLARSSL_SHA512_C)
 #include "polarssl/sha4.h"
 #endif
 #include "polarssl/dhm.h"