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>

SVN-Revision: 41872