Added mechanism to provide alternative cipher / hash implementations

All symmetric cipher algorithms and hash algorithms now include support
for a POLARSSL_XXX_ALT flag that prevents the definition of the
algorithm context structure and all 'core' functions.

include/polarssl/md2.h

@@ -3,7 +3,7 @@
  *
  * \brief MD2 message digest algorithm (hash function)
  *
- *  Copyright (C) 2006-2010, Brainspark B.V.
+ *  Copyright (C) 2006-2013, Brainspark B.V.
  *
  *  This file is part of PolarSSL (http://www.polarssl.org)
  *  Lead Maintainer: Paul Bakker <polarssl_maintainer at polarssl.org>
@@ -27,10 +27,16 @@
 #ifndef POLARSSL_MD2_H
 #define POLARSSL_MD2_H
 
+#include "config.h"
+
 #include <string.h>
 
 #define POLARSSL_ERR_MD2_FILE_IO_ERROR                 -0x0070  /**< Read/write error in file. */
 
+#if !defined(POLARSSL_MD2_ALT)
+// Regular implementation
+//
+
 /**
  * \brief          MD2 context structure
  */
@@ -74,6 +80,18 @@ void md2_update( md2_context *ctx, const unsigned char *input, size_t ilen );
  */
 void md2_finish( md2_context *ctx, unsigned char output[16] );
 
+#ifdef __cplusplus
+}
+#endif
+
+#else  /* POLARSSL_MD2_ALT */
+#include "md2_alt.h"
+#endif /* POLARSSL_MD2_ALT */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * \brief          Output = MD2( input buffer )
  *