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/sha1.c b/library/sha1.c
index 1f29a0f..a7577b4 100644
--- a/library/sha1.c
+++ b/library/sha1.c
@@ -33,6 +33,7 @@
 #if defined(MBEDTLS_SHA1_C)
 
 #include "mbedtls/sha1.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -47,11 +48,6 @@
 
 #if !defined(MBEDTLS_SHA1_ALT)
 
-/* Implementation that should never be optimized out by the compiler */
-static void mbedtls_zeroize( void *v, size_t n ) {
-    volatile unsigned char *p = (unsigned char*)v; while( n-- ) *p++ = 0;
-}
-
 /*
  * 32-bit integer manipulation macros (big endian)
  */