dtbocfg: Allow users to load devicetree overlays from userspace

[Austin Lane]

With the addition of many dev-focused and expandable devices recently
such as OpenWrt One, the Banana Pi R4 series, etc; it would be
extremely useful and beneficial to allow users to load device tree
overlays to enable additional hardware support (E.g. MikroBus cards,
Hats, etc).

I have submitted a kmod package that addresses this issue in a similar
manner to the current Raspberry Pi targets, using a configfs mount.
https://github.com/ikwzm/dtbocfg

PR: https://github.com/openwrt/packages/pull/26832

I have also created a proof-of-concept package showing how `dtbocfg`
may be consumed by other OpenWrt packages.
https://github.com/vidplace7/openwrt-dtbo/tree/main/openwrt-one-mikrobus-spidev

I welcome critique on this submission, and look forward to moving this
issue forward.

-- 
Austin Lane (vidplace7)
Packaging @ Meshtastic
https://github.com/meshtastic/openwrt