ar71xx: generate ubootenv from uci-defaults instead of providing defconfig

I noticed most of /etc/defconfig was replaced by /etc/uci-defaults, which is
indeed much nicer. So I moved over ubootenv from having a defconfig-entry to
also be generated from uci-defaults.

Signed-off-by: Daniel Golle <dgolle@allnet.de>

SVN-Revision: 28971