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.

library/x509_csr.c

@@ -39,6 +39,7 @@
 
 #include "mbedtls/x509_csr.h"
 #include "mbedtls/oid.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -60,11 +61,6 @@
 #include <stdio.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;
-}
-
 /*
  *  Version  ::=  INTEGER  {  v1(0)  }
  */