gluon-setup-mode: add gluon-enter-setup-mode

Script to enable setup mode and reboot

Fixes #2412

package/gluon-setup-mode/files/usr/bin/gluon-enter-setup-mode

+#!/bin/sh
+
+set -euo pipefail
+
+echo "Entering setup mode and rebooting..."
+uci set gluon-setup-mode.@setup_mode[0].enabled='1'
+uci commit gluon-setup-mode
+reboot