hostapd: Reintroduce Full Dynamic VLAN support

This patch brings full dynamic vlan support to netifd that existed in hostapd.sh in Attitude Adjustment.

Signed-off-by: Joseph CG Walker <Joe@ChubbyPenguin.net>
[jow@openwrt.org: changed commit message, rebased on top of current hostapd.sh]
Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

Backport of r41872

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@41963 3c298f89-4303-0410-b956-a3cf2f4a3e73