ar71xx: improve WD's My Net Wi-fi Range Extender image creation

ar71xx: improve WD's My Net Wi-fi Range Extender image creation

Previously, the generated images for the My Net Wi-fi Range Extender
wouldn't always work (and panic) due to the fixed mtd offsets and
sizes for the kernel and rootfs. This patch fixes the problem by
utilizing the shared Cybertan's partition parser to recalculate
the mtd partitions for every image dynamically everytime.

Reported-by: Pascal Paradis <peparadis@yahoo.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44665 3c298f89-4303-0410-b956-a3cf2f4a3e73