Other VLAN related options are already being processed in netifd.sh
but the vlan_file option is missing. This option allows the mapping
of vlan IDs to network interfaces and will be used in dynamic VLAN
feature for binding stations to interfaces based on VLAN
assignments. The change is done similarly to the wpa_psk_file
option.

Signed-off-by: Gong Cheng <chengg11@yahoo.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46652 3c298f89-4303-0410-b956-a3cf2f4a3e73