ar71xx: sysupgrade for My Net Wi-Fi Range Extender

ar71xx: sysupgrade for My Net Wi-Fi Range Extender

This patch enables the convenient sysupgrade feature for
WD's Wi-Fi Range Extender.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Patchwork: http://patchwork.openwrt.org/patch/4341/
[juhosg:
  - rename 'mynet_rext_check_image' function to 'cybertan_check_image'
    and move that into platform.sh,
  - remove the the model specific 'platform_do_upgrade_mynet_rext' function,
  - drop the mynet-rext.sh script,
  - remove the image/Makefile change,
  - update commit message]
[christian:
  - check all 8 bytes of the firmware/device magic]
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38966 3c298f89-4303-0410-b956-a3cf2f4a3e73