Ethernet Over USB net helper script and docs

- for CDC ECM driver

Squashed commit of the following:

commit aa9a715498e15ad46d43318a663d296f38160cf8
Author: Adam Feuer <adam@starcat.io>
Date:   Mon Feb 24 16:42:52 2020 -0800

    code formatting; removed ping

commit 56520b7f7e2b7f03697eacc2f2b5450b0f7af676
Author: Adam Feuer <adam@starcat.io>
Date:   Mon Feb 24 16:42:32 2020 -0800

    add description of netusb.sh helper script

commit afee3d33b1e66138afb7e9713d86b765ceab55b0
Author: Adam Feuer <adam@starcat.io>
Date:   Mon Feb 24 16:41:40 2020 -0800

    readme formatting

commit 772e36021f4aee2ba1df408ad29d8b9adedeed9f
Author: Adam Feuer <adam@starcat.io>
Date:   Sun Feb 23 11:38:50 2020 -0800

    updated readme, removed redundant configs

commit 082785178aa6e0d1578034b4b163785fdcb61f22
Author: Adam Feuer <adam@starcat.io>
Date:   Sat Feb 22 17:40:05 2020 -0800

    README, defconfig, and helper script improvements

tools/README.txt

@@ -787,6 +787,25 @@ mkdeps.c, cnvwindeps.c, mkwindeps.sh, and mknulldeps.sh
   eventually be solvable but for now continue to use mkwindeps.sh in
   that mixed environment.
 
+
+ netusb.sh
+ ---------
+
+    Helper script used to set up the CDC ECM Ethernet Over USB driver,
+    host routes, and IP Tables rules to support networking with a NuttX
+    system that has a CDC ECM Ethernet Over USB driver configured. Only
+    supported on Linux.
+
+    General usage:
+
+      $ ./tools/netusb.sh
+      Usage: tools/netusb.sh <main-interface> <usb-net-interface> <on|off>
+
+    This has been tested on the SAMA5D3-Xplained board; see
+    `boards/arm/sama5/sama5d3-xplained/README.txt` for more information on how
+    to configure the CDC ECM driver for that board.
+
+
 README.txt
 ----------
 

tools/netusb.sh

@@ -0,0 +1,71 @@
+#!/bin/bash
+
+#****************************************************************************
+# tools/simhostroute.sh
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.  The
+# ASF licenses this file to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance with the
+# License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+# License for the specific language governing permissions and limitations
+# under the License.
+#
+#****************************************************************************
+
+# USB Ethernet Gadget interface helper script - sets up nuttx to access
+# the internet via a host route and IP Tables.
+
+IP_NET="10.0.0.0/24"
+IP_NETMASK="255.255.255.0"
+IP_BROADCAST="10.0.0.255"
+IP_HOST="10.0.0.1"
+IP_NUTTX="10.0.0.2"
+
+if [ $# != 3 ]; then
+  echo "Usage: $0 <main-interface> <usb-net-interface> <on|off>"
+  exit 1
+fi
+
+IF_HOST=$1
+IF_USB=$2
+STATUS=$3
+
+net_off() {
+  ip route delete $IP_NET
+  ip route delete $IP_NUTTX/32
+
+  # delete nat rules to clean up
+  iptables -t nat -D POSTROUTING -o $IF_HOST -j MASQUERADE
+  iptables -D FORWARD -i $IF_HOST -o $IF_USB -m state --state RELATED,ESTABLISHED -j ACCEPT
+  iptables -D FORWARD -i $IF_USB -o $IF_HOST -j ACCEPT
+
+  ip route show
+  ifconfig $IF_USB down
+}
+
+if [ "$STATUS" == "on" ]; then
+  net_off
+  ifconfig $IF_USB up
+  ifconfig -a
+  ifconfig $IF_USB add $IP_HOST
+  ifconfig $IF_USB:0 broadcast $IP_BROADCAST netmask $IP_NETMASK
+  ip route add $IP_NET dev $IF_USB src $IP_HOST
+  ip route add $IP_NUTTX/32 dev $IF_USB src $IP_HOST
+
+  # nat to allow NuttX to access the internet
+  iptables -t nat -A POSTROUTING -o $IF_HOST -j MASQUERADE
+  iptables -A FORWARD -i $IF_HOST -o $IF_USB -m state --state RELATED,ESTABLISHED -j ACCEPT
+  iptables -A FORWARD -i $IF_USB -o $IF_HOST -j ACCEPT
+
+  ip route show
+else
+  net_off
+fi