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/md.c b/library/md.c
index 07a93ec..716c016 100644
--- a/library/md.c
+++ b/library/md.c
@@ -58,12 +58,12 @@
         POLARSSL_MD_SHA1,
 #endif
 
-#if defined(POLARSSL_SHA2_C)
+#if defined(POLARSSL_SHA256_C)
         POLARSSL_MD_SHA224,
         POLARSSL_MD_SHA256,
 #endif
 
-#if defined(POLARSSL_SHA4_C)
+#if defined(POLARSSL_SHA512_C)
         POLARSSL_MD_SHA384,
         POLARSSL_MD_SHA512,
 #endif
@@ -98,13 +98,13 @@
     if( !strcasecmp( "SHA1", md_name ) || !strcasecmp( "SHA", md_name ) )
         return md_info_from_type( POLARSSL_MD_SHA1 );
 #endif
-#if defined(POLARSSL_SHA2_C)
+#if defined(POLARSSL_SHA256_C)
     if( !strcasecmp( "SHA224", md_name ) )
         return md_info_from_type( POLARSSL_MD_SHA224 );
     if( !strcasecmp( "SHA256", md_name ) )
         return md_info_from_type( POLARSSL_MD_SHA256 );
 #endif
-#if defined(POLARSSL_SHA4_C)
+#if defined(POLARSSL_SHA512_C)
     if( !strcasecmp( "SHA384", md_name ) )
         return md_info_from_type( POLARSSL_MD_SHA384 );
     if( !strcasecmp( "SHA512", md_name ) )
@@ -133,13 +133,13 @@
         case POLARSSL_MD_SHA1:
             return &sha1_info;
 #endif
-#if defined(POLARSSL_SHA2_C)
+#if defined(POLARSSL_SHA256_C)
         case POLARSSL_MD_SHA224:
             return &sha224_info;
         case POLARSSL_MD_SHA256:
             return &sha256_info;
 #endif
-#if defined(POLARSSL_SHA4_C)
+#if defined(POLARSSL_SHA512_C)
         case POLARSSL_MD_SHA384:
             return &sha384_info;
         case POLARSSL_MD_SHA512: