Support for D-Link DIR842 (Rev C3)

[s_2]

Thanks, so I will only include the EU signature for C3.
Even if there were a non-EU C3 version, it could still be flashed via the recovery just like before.

Now I'm just waiting for someone to test DIR-859, since I do not own that device:

D-Link DIR-859 A3 AC1750 works with DIR-869 A1 image

The latest firmware versions from D-Link will no longer accept the factory.bin format currently used by OpenWRT via the normal update procedure, so you'd have to flash via the emergency recovery, which only works with older browsers. I'm trying to add support for the 'new style' image that can be flashed directly from the D-Link Web UI: As I do not own DIR-859 myself, I could only verify image generation against the official firmware images available from D-Link, and it looks good so far. H…