IPQ40XX: Introduce Kernel Flag

[Nick]

The current ipq40xx network stack does no longer work for a few targets:
- Zyxel
- Fritz!Box 4040
- Fritz!Box 7530
- ZyXEL NBG6617
- GL-B1300
- ...

Can we please finally add my PR? I introduced a kernel flag that is 
about switching between the different options
1. Port Isolation
2. Nested VLANs
ith this I hope to satisfy both sides.

Here is the PR:
https://github.com/openwrt/openwrt/pull/3596