ipq40xx: RT-AC58U: "easy install" factory.trx

ipq40xx: RT-AC58U: "easy install" factory.trx

This patch adds a image-builder compatible factory install
image that can be flashed directly through both the WebUI or u-boot.

Installation Instructions for the -factory.trx image:
- For the web-console you just upload the .trx file.
  1. login into your router
  2. Select "Administration" under "Advanced Settings".
  3. Select the "Firmware Upgrade" tab
  4. Click on the "Choose File" button and select the
     firmware file.
     (If the web-interface doesn't accept the file try
      renaming it to: FW_RT-AC58U_3.1.1.1.trx)

  5. Hit "Upload" Button

- Place the -factory.trx file into your TFTP-server's
  root-directory and rename it to RT-AC58U.trx. Connect the serial
  cable and power up the router. In the serial console boot menu prompt
  you have to press the 2-key to choose:
   "2: Load System code then write to Flash via TFTP."
  and then follow the instruction on the serial console.

After the image is written to the NAND, the router will automatically
reboot into a modified u-boot that flashes the packed image before
it reboots one more time into the new OpenWrt installation.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>