Add host and device scripts for USB networking

debian/librem5-devkit-host.install

 scripts/librem5-devkit-flash-image /usr/bin/
+scripts/librem5-usbnet /usr/bin/

scripts/librem5-usbnet

+#!/bin/bash
+#
+# Copyright (C) 2019 Purism SPC
+#
+# SPDX-License-Identifier: GPL-3.0+
+#
+
+set -e
+
+if [ $# -ne 2 ]; then
+    echo "Usage: librem5-usbnet <IPv4 address> <network interface>"
+    exit
+fi
+
+ip addr add $1/24 dev $2
+sysctl net.ipv4.ip_forward=1

test_scripts/usbnet

+#!/bin/bash
+#
+# Copyright (C) 2019 Purism SPC
+#
+# SPDX-License-Identifier: GPL-3.0+
+#
+
+set -e
+
+if [ $# -ne 0 ]; then
+    echo "Usage: usbnet"
+    exit
+fi
+
+ip addr add 192.168.42.2/24 dev usb0
+ip route add default via 192.168.42.1 dev usb0