Add exit device to SNAT

[Tbaile]

While it's possible to allow a SNAT to redirect through a specific zone, in some configuration it's useful to have a SNAT through a specific device. (for instance through a specific RED) This is cause if the link drops, you can overcome the SNAT using a different device.

This can be done in UCI, we just need the UI to do so.

Ref:

• UCI: https://git.openwrt.org/?p=project/firewall3.git;a=blob;f=snats.c;hb=HEAD#l29

[Tbaile]

It appears that setting the device to what you need in the SNAT firewall entry will enable the correct behaviour. And it's not overridden by any config change through the UI. I'll check with support to figure out if it's something absolutely necessary through the UI.

[Tbaile]

QA: Check with latest dev if everything checks out