bridge-vlan: add support for defining aliases for vlan ids

bridge-vlan: add support for defining aliases for vlan ids

When defining a bridge-vlan like this:

config bridge-vlan
	option device 'switch0'
	option vlan '1'
	option ports 'lan1 lan2 lan3 lan4'
	option alias 'lan'

You can use switch0.lan instead of switch0.1 to refer to the VLAN.
This ensures that the VLAN ID can be kept in a single place in the config

Signed-off-by: Felix Fietkau <nbd@nbd.name>