gluon-core: fix missing ifname for mesh_radio networks in uci

[lemoer]

Before this comit, the networks in /etc/config/network looked like this:

config interface 'mesh_radio0'
    	option proto 'gluon_mesh'

After this patch, the networks look like this:

config interface 'mesh_radio0'
        option proto 'gluon_mesh'
    	option ifname 'mesh0'

Especially, this fixes a regression bug, that the "wifi" section in the
respondd request type "neighbours" was empty:

~# gluon-neighbour-info -d ::1 -r neighbours | ffh_pretty_json
{
  "wifi": [
  ],
  ...
}

After this commit, the section (correctly) looks like this:

root@UFU-FWH-A272-Tresckowstr-GemR-vorne:~# gluon-neighbour-info -d ::1 -r neighbours | ffh_pretty_json
{
  "wifi": {
    "ca:38:7e:42:5f:21": {
      "neighbours": {
        "fe:9f:4d:01:ea:e1": {
          "noise": -102,
          "inactive": 50,
          "signal": -84
        },
        "fe:df:b9:84:37:51": {
          "noise": -102,
          "inactive": 20,
          "signal": -73
        }
      }
    }
  },
  ...
}

The corresponding code can be found here: https://github.com/freifunk-gluon/gluon/blob/master/package/gluon-respondd/src/respondd-neighbours.c#L99-L101

[lemoer]

I found the commit, that originally introduced this regression:

0f1fa24#diff-727ba88a65d0c44d3295d11b2dc2a04bd341ab16a978f4c1af6a3b09140921fd

[lemoer]

I think, this PR is patching the wrong side of this issue.

[lemoer]

Closed in favor of #2599 .