ath79: create APBoot-compatible image for Aruba AP-175

ath79: create APBoot-compatible image for Aruba AP-175

As was done in commit e11d00d44c66 ("ath79: create Aruba AP-105 APBoot
compatible image"), alter the Aruba AP-175 image generation process so
OpenWrt can be loaded with the vendor Aruba APBoot. Since the
remainder of the explanation and installation process is identical,
continuing the quote from that commit:

This works by prepending the OpenWrt LZMA loader to the uImage and
jumping directly to the loader. Aruba does not offer bootm on these
boards.

This approach keeps compatibility to devices which had their U-Boot
replaced. Both bootloaders can boot the same image.

With this patch, new installations do not require replacing the
bootloader and can be performed from the serial console without
opening the case.

Installation
------------

1. Attach to the serial console of the AP-175.
   Interrupt autoboot and change the U-Boot env.

   $ setenv apb_rb_openwrt "setenv ipaddr 192.168.1.1;
     setenv serverip 192.168.1.66;
     netget 0x84000000 ap175.bin; go 0x84000040"
   $ setenv apb_fb_openwrt "cp.b 0xbf040000 0x84000000 0x10000;
     go 0x84000040"
   $ setenv bootcmd "run apb_fb_openwrt"
   $ saveenv

2. Load the OpenWrt initramfs image on the device using TFTP.
   Place the initramfs image as "ap175.bin" in the TFTP server
   root directory, connect it to the AP and make the server reachable
   at 192.168.1.66/24.

   $ run apb_rb_openwrt

3. Once OpenWrt booted, transfer the sysupgrade image to the device
   using scp and use sysupgrade to install the firmware.

Signed-off-by: Martin Kennedy <hurricos@gmail.com>