alfred: Support interfaces IDs with more than two digits

Occationally /proc/net/if_inet6 contains interface IDs with
three digits. In this case, the regex in wait_for_ll_address()
does not work anymore and alfred is not starting.

This patch changes the evaluation so that fields are used instead
of the mere position by counting characters.

This is a backport from openwrt-routing and can be removed when
updating to OpenWRT 18.06, but not for newer versions of 17.01.

Fixes #98.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhgammer <rlanghammer@web.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>