libtomcrypt: implement zeromem() with memzero_explicit()

This change is part of commit 03121b2 ("core: crypto: libtomcrypt:
fix LTC_CLEAN_STACK bug"). LTC_CLEAN_STACK will be disabled in a later
commit because it is not supported anymore upstream ("LTC_CLEAN_STACK
is considered as broken"), so we keep only the zeromem() part of the
commit.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

core/lib/libtomcrypt/src/misc/zeromem.c

@@ -1,6 +1,7 @@
 /* LibTomCrypt, modular cryptographic library -- Tom St Denis */
 /* SPDX-License-Identifier: Unlicense */
 #include "tomcrypt_private.h"
+#include <string_ext.h>
 
 /**
    @file zeromem.c
@@ -14,9 +15,6 @@
 */
 void zeromem(volatile void *out, size_t outlen)
 {
-   volatile char *mem = out;
    LTC_ARGCHKVD(out != NULL);
-   while (outlen-- > 0) {
-      *mem++ = '\0';
-   }
+   memzero_explicit((void *)out, outlen);
 }