<a href="https://openwrt.org/">OpenWrt</a>.
<blockquote>OpenWrt is described as a Linux distribution for embedded devices.

Instead  of trying to create a single, static firmware, OpenWrt provides a fully  writable filesystem with package management. This frees you from the  application selection and configuration provided by the vendor and  allows you to customize the device through the use of packages to suit  any application. For developer, OpenWrt is the framework to build an  application without having to build a complete firmware around it; for  users this means the ability for full customization, to use the device  in ways never envisioned.</blockquote>
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This sounds very useful.