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/md.c b/library/md.c
index 00249af..c54ae85 100644
--- a/library/md.c
+++ b/library/md.c
@@ -33,6 +33,7 @@
 
 #include "mbedtls/md.h"
 #include "mbedtls/md_internal.h"
+#include "mbedtls/utils.h"
 
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
@@ -48,11 +49,6 @@
 #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;
-}
-
 /*
  * Reminder: update profiles in x509_crt.c when adding a new hash!
  */