[PATCH] base-files: simplify setting device MAC

[Evgeny K]

> 3. Drop section name
>    It's not required by netifd or LuCI & it's not needed by this script
>    as $device contains a single device name now.

It could be convenient to have it, though. In the former scheme of
things, if one would like to know the interface behind, say, lan it
was possible to do with just 'uci -q get network.lan.ifname`. Now it
has become pretty complicated. Adding a name to the device section
would allow one to at least do it in 2 steps: `uci -q get
network.lan.device` and `uci -q get network.$device.ports`.