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.

library/xtea.c

@@ -1,7 +1,7 @@
 /*
  *  An 32-bit implementation of the XTEA algorithm
  *
- *  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>
@@ -29,6 +29,8 @@
 
 #include "polarssl/xtea.h"
 
+#if !defined(POLARSSL_XTEA_ALT)
+
 /*
  * 32-bit integer manipulation macros (big endian)
  */
@@ -160,6 +162,7 @@ int xtea_crypt_cbc( xtea_context *ctx,
 
     return( 0 );
 }
+#endif /* !POLARSSL_XTEA_ALT */
 
 #if defined(POLARSSL_SELF_TEST)