Add mac-telnet - a layer 2 Telnet/SSH server useful to access bricked devices

This package adds the mac-telnet server, client, ping and discovery utilities.
See https://github.com/aouyar/MAC-Telnet for details.

This package uses the fork from https://github.com/jow-/MAC-Telnet as source,
the code there has a number of bugfixes and results in smaller binaries since
most core functionality has been ported to libubox facilities provided by
OpenWrt.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>