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/ssl_tls.c b/library/ssl_tls.c
index e8063d2..84f9c77 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -46,6 +46,7 @@
 #include "mbedtls/debug.h"
 #include "mbedtls/ssl.h"
 #include "mbedtls/ssl_internal.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -53,11 +54,6 @@
 #include "mbedtls/oid.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;
-}
-
 /* Length of the "epoch" field in the record header */
 static inline size_t ssl_ep_len( const mbedtls_ssl_context *ssl )
 {