ath79: build Compex WPJ531 cpximg files

ath79: build Compex WPJ531 cpximg files

Generate additional images that are compatible to the cpximg loader.
The cpximg loader can be started either by holding the reset button during
power up or by entering the u-boot prompt and entering 'cpximg'.
Once it's running, a TFTP-server under 192.168.1.1 will accept the image
appropriate for the board revision that is etched on the board.

For example, if the board is labelled '7A04':

tftp -v -m binary 192.168.1.1 -c put openwrt-ath79-generic-compex_wpj531-16m-squashfs-cpximg-7A04.bin

These files can also be used with the sysupgrade utility in stock images (add
SSH key in luci for root access).

Signed-off-by: Leon M. George <leon@georgemail.eu>
[fix sorting of definitions]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>