ar71xx: dragino2: add missing sysupgrade support.

The original dragino2 board support was missing some changes from the
upstream svn repository (http://svn.dragino.com/dragino2

) that supported
sysupgrade.

[juhosg: change subject]

Signed-off-by: Karl Palsson <karlp@remake.is>
Patchwork: http://patchwork.openwrt.org/patch/4801/


Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39429