dsaconfig: introduce package for UCI configuration of VLAN filter rules

dsaconfig: introduce package for UCI configuration of VLAN filter rules

This package provides the necessary files to translate `config dsa_vlan`
and `config dsa_port` sections  of `/etc/config/network` into appropriate
bridge vlan filter rules.

The approach of the configuration is to bridge all DSA ports into a logical
bridge device, called "switch0" by default, and to set VLAN port membership,
tagging state and PVID as specified by UCI on each port and on the switch
bridge device itself, allowing logical interfaces to reference port VLAN
groups by using "switch0.N" as ifname, where N denotes the VLAN ID.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>