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.
(cherry picked from commit 4087c47043cb7b8b51e69f1de47ab6a2bccead3d)

include/polarssl/sha4.h

@@ -3,7 +3,7 @@
  *
  * \brief SHA-384 and SHA-512 cryptographic 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>
@@ -27,6 +27,8 @@
 #ifndef POLARSSL_SHA4_H
 #define POLARSSL_SHA4_H
 
+#include "config.h"
+
 #include <string.h>
 
 #if defined(_MSC_VER) || defined(__WATCOMC__)
@@ -39,6 +41,10 @@
 
 #define POLARSSL_ERR_SHA4_FILE_IO_ERROR                -0x007A  /**< Read/write error in file. */
 
+#if !defined(POLARSSL_SHA1_ALT)
+// Regular implementation
+//
+
 /**
  * \brief          SHA-512 context structure
  */
@@ -83,6 +89,18 @@ void sha4_update( sha4_context *ctx, const unsigned char *input, size_t ilen );
  */
 void sha4_finish( sha4_context *ctx, unsigned char output[64] );
 
+#ifdef __cplusplus
+}
+#endif
+
+#else  /* POLARSSL_SHA4_ALT */
+#include "sha4_alt.h"
+#endif /* POLARSSL_SHA4_ALT */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * \brief          Output = SHA-512( input buffer )
  *