hw/omnia/serial-boot: Mention that upload of kwboot image over serial fails when another console is connected

I wanted to send a PR, but apparently I'm not allowed to create any personal repos on this gitlab.

## Booting

For the purpose of this tutorial, it is assumed that you have all the binaries
in your current working directory and that your serial line is _ttyUSB0_.

1. Disconnect your Omnia from power supply.
2. Run the following command.

    ```
    kwboot -t -b uboot /dev/ttyUSB0
    ```

3. Connect the power supply.

    kwboot will trigger the _boot over serial_ mode of your Omnia automatically

4. get ready to interrupt the booting process as described below

If everything works well, you should see the U-Boot being transferred to your
Omnia and once done, you'll see U-Boot prompt and you can use that to recover
your device.

+ Close all other console connections to the router while booting. If you have
+ another serial console open and attached to the `/dev/ttyUSB0`, the upload of
+ the boot image will be very slow and will not be able to get uploaded in time.
+ Instead, you can see the router rebooting itself in the middle of the upload.
+
Sometimes the `kwboot` command fails on the first try, but you can just run it
again and again till it succeeds.

You may append additional argument `-B 1500000` to increase transfer speed.
The default speed is 115200 bauds/sec. Omnia supports speeds up to the 5200000
bauds/sec, but not all serial cables can handle such high speed modes.