scripts: introduce a new helper script glide-update.sh

Introduce a new helper script glide-update.sh to be able to easily
update glide vendors.
This script was originally taken from rkt.

Author: Dongsu Park

scripts/glide-update.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+#
+# Update vendored dedendencies.
+#
+set -e
+
+if ! [[ "$0" =~ "scripts/glide-update.sh" ]]; then
+	echo "must be run from repository root"
+	exit 255
+fi
+
+if [ -z "$(command -v glide)" ]; then
+	echo "glide: command not found"
+	exit 255
+fi
+
+if [ -z "$(command -v glide-vc)" ]; then
+	echo "glide-vc: command not found"
+	exit 255
+fi
+
+glide update --strip-vcs --strip-vendor --update-vendored --delete
+glide vc --only-code --no-tests