kernel: netconsole: add network console logging support

Accessing the console on many devices is difficult.
netconsole eases debugging on devices that crash
after the network is up.

Reference to the netconsole documentation in upstream Linux:
<https://www.kernel.org/doc/html/latest/networking/netconsole.html>
|
|netconsole=[+][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
|
| where
|  +            if present, enable extended console support
|  src-port     source for UDP packets (defaults to 6665)
|  src-ip       source IP to use (interface address)
|  dev          network interface (eth0)
|  tgt-port     port for logging agent (6666)
|  tgt-ip       IP address for logging agent
|  tgt-macaddr  ethernet MAC address for logging agent (broadcast)

OpenWrt specific notes:

OpenWrt's device userspace scripts are attaching the network
interface (i.e. eth0) to a (virtual) bridge (br-lan) device.
This will cause netconsole to report:
|network logging stopped on interface eth0 as it is joining a master device
(and unfortunately the traffic/logs to stop at this point)

As a workaround, the netconsole module can be manually loaded
again after the bridge has been setup with:

 insmod netconsole netconsole=@/br-lan,@192.168.1.x/MA:C...

One way of catching errors before the handoff, try to
append the /etc/modules.conf file with the following extra line:
 options netconsole netconsole=@/eth0,@192.168.1.x/MA:C...

and install the kmod-netconsole (=y) into the base image.

Signed-off-by: Catalin Toda <catalinii@yahoo.com>
(Added commit message from PR, added links to documentation)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

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