Add wiki page on how to configure AX.25 (`ax25.engine`)

[martinhpedersen]

As of v0.14.0 we're adding the concept AX.25 engine, allowing the user to configure which transport package to be used for handling the ax25:// connection scheme and ax25 listener.

1. We need to update the existing wiki page AX.25 on Linux to reflect these changes.
2. We should add a new page explaining the general concept including some example config for various setups.
3. I'd also love to see a step-by-step guide (much like AX.25 on Linux) on how to configure Pat+Direwolf using the agwpe engine. This is by far the easiest and most flexible solution for most users.

Here are some rough (untested) examples to get us started:

agpwe

AGWPE / Direwolf. Recommended for most users using software TNC. Works on all platforms.

{
  "listen": ["ax25"],
  "ax25": {
    "engine": "agwpe",
    "beacon": {"every": 3600, "message": "Winlink P2P", "destination": "IDENT"}
  },
  "agwpe": {"addr": "localhost:8000", "radio_port": 0}
}

linux

Linux kernel (aka "libax25"). Recommended for users with hardware TNCs supporting KISS mode.

{
  "ax25": {
    "engine": "linux",
    "beacon": {"every": 3600, "message": "Winlink P2P", "destination": "IDENT"}
  },
  "ax25_linux": {"port": "wl2k"}
}

serial-tnc

Serial TNCs (Kenwood TM-D7x0/TH-D7x, kantronics etc). Last resort for users with hardware TNCs without KISS mode.

Note: Listener and beaconing not supported by this engine.

{
  "ax25": {
    "engine": "serial-tnc"
  },
  "serial-tnc": {
    "path": "/dev/ttyUSB0",
    "serial_baud": 57600,
    "hbaud": 1200,
    "type": "kenwood"
  }
}

[martinhpedersen]

As of v0.14.0 the default config will reflect these changes. On Linux, the default config will be set to linux. On all other platforms, agwpe will be the default engine.

[martinhpedersen]

I've started on a wiki page explaining how to configure Pat for Direwolf here: https://github.com/la5nta/pat/wiki/AX.25-with-Direwolf