Add support to cache IPK packages for offline install

Why a Spare Router.md

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+# Why a "Spare Router" Configuration?
+
+Many people who use OpenWrt wind up with unused routers when
+they retire one for a newer device.
+These are perfectly functioning devices that are perhaps older
+or missing a certain function.
+
+They could be easily re-used and passed along to friends, family or neighbors.
+BUT... you have to solve a couple problems:
+
+1. The router retains all your personal info:
+  passwords, certificates, idiosyncratic packages. etc.
+2. You can't remember how it was configured, so you
+  can't even connect to it.
+
+**The remedy:** A "spare router" configuration script that
+you can use when you take a router out of service.
+It leaves the router with current OpenWrt firmware
+configured for Wifi access, and
+a known useful set of package to make it easy to reuse.
+The script also prints a label that you can attach to
+the router so that you can get started quickly the next
+time you get it out.
+
+## Usage
+
+When you retire a router from service, run this script.
+It is available at
+[config-spare-router.sh](https://github.com/richb-hanover/OpenWrtScripts/blob/master/config-spare-router.sh)
+To use it:
+
+* Connect your laptop via Ethernet to a LAN port
+* Connect the router's WAN Ethernet to the Internet
+  (perhaps the LAN port of the new router).
+* Use the LuCI GUI to reset settings to
+  factory default (**System -> Backup/Flash firmware**)
+* Use the [Firmware Selector](https://firmware-selector.openwrt.org/)
+  to find and flash the latest firmware
+* SSH into the router using the Ethernet connection
+* Run the attached script (see the instructions within it)
+* The script prints a label: cut it out and tape it to the router
+* _Pro tip:_ Printing the label in 12-point type produces a 
+  "business card" size label.
+* _Pro tip:_ Snip the model number from the paper
+  and tape it directly to the power brick.
+  (Use the text from the "Power Brick Label" section.)
+* _Pro tip:_ Place the router and its power brick in a Ziploc bag
+  to keep them together.
+
+The `config-spare-router.sh` script may be run multiple times without bad effect.
+When the script completes, it displays configuration similar to this,
+suitable for printing and taping to the router.
+
+```text
+# ======= Printed with: print-router-label.sh =======
+#      Device: Linksys E8450 (UBI)
+#     OpenWrt: OpenWrt 23.05.5 r24106-10cc5fcd00
+#  Connect to: http://SpareRouter.local
+#          or: ssh root@SpareRouter.local
+#         LAN: 172.30.42.1
+#        User: root
+#    Login PW: SpareRouter
+#   Wifi SSID: SpareRouter
+#     Wifi PW: <no password>
+#  Configured: 2024-11-28
+# === See github.com/richb-hanover/OpenWrtScripts ===
+#
+# Label for Power Brick: Linksys E8450 (UBI)
+```
+
+## When you (re)deploy the router
+
+The default settings are (intentionally) insecure.
+Remember to change the following:
+
+* Root password (**System -> Administration**)
+* Wifi credentials (**Network -> Wireless**)
+* Enable other Wifi radios (**Network -> Wireless**)
+* Change the LAN interface IP address and other settings as needed (**Network -> Interfaces**)
+* (Optional) Configure SQM (**Network -> SQM QoS**)
+* (Optional) Change the hostname (**System -> System**)
+* (Optional) Install other packages as needed
+* (Optional) Travelmate (**Services -> Travelmate**)
+  Click the **Interface Wizard** button one time
+* (Optional) Re-run the `print-router-label.sh` to create
+  a new label and tape it to the router, so you don't
+  have to fuss the next time you work on it.
+
+## Rationale for the configuration choices
+
+This script was designed for ease of use.
+It presumes that it is being installed on a modern (post-2021)
+router that has plenty of RAM and Flash storage, so that size
+was not a consideration.
+
+* **Root password:** To make it easy to re-use the router,
+  the `root` password is set to `SpareRouter`.
+  There is no need for strong security here, as you will be changing
+  the password when you set it up in its new location.
+* **LAN Address:** The LAN IP address is set to `172.30.42.1`.
+  This is a
+  [valid private IP address range](https://en.wikipedia.org/wiki/Private_network)
+  (like `10...` and `192.168...` subnets) but it is less commonly used.
+  This means that you can bring the router into virtually any
+  network environment without concern for IP address conflicts,
+  then use the LuCI GUI to configure the LAN.
+* **Hostname:** is set to "SpareRouter".
+  Because `umdns` is installed, you can connect using
+  `http://SpareRouter.local` or `ssh root@SpareRouter.local`
+  no matter what the LAN IP address is.
+* **Wifi settings:** The SSID is of the _first_ radio is set
+  to `SpareRouter` without encryption.
+  No other radios are enabled.
+  As with the root password, there is no need for a strong password,
+  because you will be changing it immediately.
+* **Time Zone:** As a convenience, the time zone is set to `Americas/New York`.
+  You can use the LuCI GUI to re-configure as needed.
+* **Software packages:** The script installs a minimal set of useful
+  packages that are required to bootstrap a new router.
+
+  * **luci** Released versions of OpenWrt already install `luci`.
+    Re-installing does no harm.
+  * **umdns** Allows the router to advertise its name as "SpareRouter"
+    (e.g., connect using `ssh root@sparerouter.local`)
+  * **luci-app-sqm** All OpenWrt routers ought to have the SQM package
+    installed to minimize bufferbloat. Just do it.
+  * **travelmate** _and_
+  * **luci-app-travelmate** This packages allow a router to
+    act as a Wifi repeater by making a wireless "uplink"
+    to an existing network
+    Even if there's no Ethernet connection for the spare routers's WAN port,
+    you can use the wireless uplink to download additional packages.
+
+## Modifications
+
+This script provides a stable platform for re-deploying old routers.
+Feel free to make suggestions (create an Issue) for _minimal_ tweaks that
+would improve the script. Enjoy!
+
+## Old information
+
+The script also has a large number of lines that are commented out.
+These were steps for other packages that are not essential for a "Spare Router".
+Feel free to experiment with these sections in your own copy of the script.