Fixes site.conf checks.

This might improve stability on the TL-WR841 v9.

hostapd would switch the primary and secondary channel on 5GHz networks in
certain circumstances, completely breaking the adhoc interfaces of the WLAN
adapter (they would lose their configuration).

As a temporary fix, disable this channel switch function.

Original patch by @MPW1412

The TL-WR741ND will identify itself as a TL-WR740N/ND as well. This is fine as
the images are identical anyways.

Recent changes in the routing configuration of OpenWrt broke source address
selection, sometimes leading to the node trying to contact other addresses in
the mesh network from the next-node address.

Revert the problematic commits until this has been solved upstream.

This conflicts with our reset button usage.

With a backported patch from the OpenWrt trunk, it is now easy to generate an
equivalent configuration using CONFIG_ALL_KMODS.

The build will take a bit longer because all kernel module packages are actually
built even when they are not included in the image, but adding new targets
becomes a lot easier.

Also, related documentation updates and fixes.

Also add some patches from OpenWrt trunk and use the CT ath10k firmware.