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)

library/md2.c

@@ -1,7 +1,7 @@
 /*
  *  RFC 1115/1319 compliant MD2 implementation
  *
- *  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>
@@ -39,6 +39,8 @@
 #include <stdio.h>
 #endif
 
+#if !defined(POLARSSL_MD2_ALT)
+
 static const unsigned char PI_SUBST[256] =
 {
     0x29, 0x2E, 0x43, 0xC9, 0xA2, 0xD8, 0x7C, 0x01, 0x3D, 0x36,
@@ -163,6 +165,8 @@ void md2_finish( md2_context *ctx, unsigned char output[16] )
     memcpy( output, ctx->state, 16 );
 }
 
+#endif /* !POLARSSL_MD2_ALT */
+
 /*
  * output = MD2( input buffer )
  */