Replace memset() with mbedtls_platform_memset()

Steps:

1. sed -i 's/\bmemset(\([^)]\)/mbedtls_platform_memset(\1/g' library/*.c tinycrypt/*.c include/mbedtls/*.h scripts/data_files/*.fmt

2. Manually edit library/platform_util.c to revert to memset() in the
implementations of mbedtls_platform_memset() and mbedtls_platform_memcpy()

3. egrep -n '\<memset\>' library/*.c include/mbedtls/*.h tinycrypt/*.c
The remaining occurrences are in three categories:
    a. From point 2 above.
    b. In comments.
    c. In the initialisation of memset_func, to be changed in a future commit.
diff --git a/library/x509.c b/library/x509.c
index d570f71..33dc0b9 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -650,7 +650,7 @@
                            mbedtls_x509_name *cur )
 {
     mbedtls_x509_buf_raw name_buf = { p, len };
-    memset( cur, 0, sizeof( mbedtls_x509_name ) );
+    mbedtls_platform_memset( cur, 0, sizeof( mbedtls_x509_name ) );
     return( mbedtls_x509_name_cmp_raw( &name_buf, &name_buf,
                                        x509_get_name_cb,
                                        &cur ) );
@@ -943,7 +943,7 @@
     const char *short_name = NULL;
     char s[MBEDTLS_X509_MAX_DN_NAME_SIZE], *p;
 
-    memset( s, 0, sizeof( s ) );
+    mbedtls_platform_memset( s, 0, sizeof( s ) );
 
     name = dn;
     p = buf;