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/pkcs12.c b/library/pkcs12.c
index c603a13..98b8324 100644
--- a/library/pkcs12.c
+++ b/library/pkcs12.c
@@ -36,6 +36,7 @@
 #include "mbedtls/pkcs12.h"
 #include "mbedtls/asn1.h"
 #include "mbedtls/cipher.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -47,11 +48,6 @@
 #include "mbedtls/des.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;
-}
-
 static int pkcs12_parse_pbe_params( mbedtls_asn1_buf *params,
                                     mbedtls_asn1_buf *salt, int *iterations )
 {