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.
diff --git a/library/camellia.c b/library/camellia.c
index 2bc3443..bb87875 100644
--- a/library/camellia.c
+++ b/library/camellia.c
@@ -1,7 +1,7 @@
 /*
  *  Camellia 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>
@@ -35,6 +35,8 @@
 
 #include "polarssl/camellia.h"
 
+#if !defined(POLARSSL_CAMELLIA_ALT)
+
 /*
  * 32-bit integer manipulation macros (big endian)
  */
@@ -656,6 +658,7 @@
     return( 0 );
 }
 #endif /* POLARSSL_CIPHER_MODE_CTR */
+#endif /* !POLARSSL_CAMELLIA_ALT */
 
 #if defined(POLARSSL_SELF_TEST)