KISS TNC Support

[jmcblane]

KISSTNCsupport.patch

Here's a patch that adds KISS TNC support. It will now connect directly to the VGC VR-N7600 without the need of direwolf and is fully functional. GPT-5.4 one-shotted the implementation.

[ben-kuhn]

Yes, please add this. Now that KISS and AX.25 support is being dropped from the Linux kernel, there is no way to use modern hardware TNCs since they don't speak AGWPE. ldsped, as a workaround for this, is done.