tools: add tool for generating random hexadecimal values

This tool generates a random hexadecimal value of a given maximum size.
This is useful for generating random canary values during compile-time
for the ssp module which currently uses a constant value.

dist/tools/randhex/README.md

@@ -0,0 +1,7 @@
+randhex.py
+----------
+
+Usage: `randhex.py <MAXBITS>`
+
+This script generates a random hexadecimal value of the given maximum
+size in bits. If `MAXBITS` is not specified it defaults to 64.

dist/tools/randhex/randhex.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+
+#
+# Copyright (C) 2020 Sören Tempel <tempel@uni-bremen.de>
+#
+# This file is subject to the terms and conditions of the GNU Lesser
+# General Public License v2.1. See the file LICENSE in the top level
+# directory for more details.
+#
+
+import sys
+import secrets
+
+if len(sys.argv) >= 2:
+    maxbits = int(sys.argv[1])
+else:
+    maxbits = 64
+
+maxval = 2 ** maxbits
+print(hex(secrets.randbelow(maxval)))