Cleanup up non-prototyped functions (static) and const-correctness

More fixes based on the compiler directives -Wcast-qual -Wwrite-strings
-Wmissing-prototypes -Wmissing-declarations. Not everything with regards
to -Wcast-qual has been fixed as some have unwanted consequences for the
rest of the code.
diff --git a/library/entropy.c b/library/entropy.c
index 9662454..253ae3d 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -1,7 +1,7 @@
 /*
  *  Entropy accumulator implementation
  *
- *  Copyright (C) 2006-2011, 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>
@@ -81,14 +81,14 @@
 /*
  * Entropy accumulator update
  */
-int entropy_update( entropy_context *ctx, unsigned char source_id,
-                    const unsigned char *data, size_t len )
+static int entropy_update( entropy_context *ctx, unsigned char source_id,
+                           const unsigned char *data, size_t len )
 {
     unsigned char header[2];
     unsigned char tmp[ENTROPY_BLOCK_SIZE];
     size_t use_len = len;
     const unsigned char *p = data;
-   
+
     if( use_len > ENTROPY_BLOCK_SIZE )
     {
         sha4( data, len, tmp, 0 );
@@ -102,7 +102,7 @@
 
     sha4_update( &ctx->accumulator, header, 2 );
     sha4_update( &ctx->accumulator, p, use_len );
-    
+
     return( 0 );
 }