ipq40xx: add cpximg to flash Compex WPJ428 via bootloader

ipq40xx: add cpximg to flash Compex WPJ428 via bootloader

Generate a cpximg that is compatible with the cpximg loader in Compex' u-boot.
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 an image
appropriate for the board revision that is etched on the board (e.g. 6A04).

The image can be pushed using tftp:
  tftp -v -m binary 192.168.1.1 -c put openwrt-ipq40xx-generic-compex_wpj428-squashfs-cpximg-6a04.bin

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

In mkmylofw_32m, the calculation of the "partition size" has been preferred
to just padding the partition as this will result in less block transfers
during flashing (while the additional complexity is bearable).

Signed-off-by: Leon M. George <leon@georgemail.eu>
Co-developed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>