cjdns: fix uci-defaults (#714)

cjdns: fix uci-defaults (#714)

Since https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=4b9a67362d70c544b85078b8d5c661f43f7472d9,
uci network config interface sections use 'device' instead of 'ifname'

Even if 'device' seems to always be set by
package/base-files/files/bin/config_generate, we keep the fallbacks
in case someone install the package via opkg and has a non migrated
network config

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
(cherry picked from commit 383c52ff8d9c0577a262d014aca66bd4095f4e38)