[qmp-system] Fix #377 (1): backport QinQ branch into testing
This commit fixes issue #377, regarding the ways VLANs are specified in /etc/config/network. The previous method (with @wlan0-like stuff) was never meant to work, although it just happened to work. Recent OpenWrt changes have deprecated that *unsupported* way to declare VLANs. Backporting the work in branch QinQ and adding a couple of changes (e.g. linklayer details for BMX6) fixes the problem.