Remove individual copies of mbedtls_zeroize()

This commit removes all the static occurrencies of the function
mbedtls_zeroize() in each of the individual .c modules. Instead the
function has been moved to utils.h that is included in each of the
modules.
diff --git a/library/entropy.c b/library/entropy.c
index e17512e..37fdf3a 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -35,6 +35,7 @@
 
 #include "mbedtls/entropy.h"
 #include "mbedtls/entropy_poll.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -59,11 +60,6 @@
 #include "mbedtls/havege.h"
 #endif
 
-/* Implementation that should never be optimized out by the compiler */
-static void mbedtls_zeroize( void *v, size_t n ) {
-    volatile unsigned char *p = v; while( n-- ) *p++ = 0;
-}
-
 #define ENTROPY_MAX_LOOP    256     /**< Maximum amount to loop before error */
 
 void mbedtls_entropy_init( mbedtls_entropy_context *ctx )