adds optional script to enable poe passthrough on cpe210

The script is only included in ar71xx builds and only executes on the
cpe210 v1.1 hardware.
It is intended to be called from /etc/rc.local.fff_userconfig since the
state of poe passthrough is reset on each reboot.

Signed-off-by: Tobias Klaus <tk+ff@meskal.net>
Reviewed-by: Christian Dresel <fff@chrisi01.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>