Use the newly introduced devtype attribute for more robust network device
type detection. This also allows us to easily recognize DSA ports.
Furthermore, synthesize VLAN devices declared by uci bridge-vlan sections,
similar to how it is done for legacy swconfig switch_vlan ones.
Finally implement a new Network.Device.getParent() method to use the newly
available "parent" attribute to resolve the base device of DSA ports or
VLAN devices.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>