fix: anygw fails to configure on some dsa devices

some devices using dsa, don't have an bridge called br-lan
instead it is present a configuration like

```
config device 'switch'
	option name 'switch'
	option type 'bridge'

config bridge-vlan 'lan_vlan'
	option device 'switch'
	option vlan '1'
	option ports 'lan1 lan2 lan3 lan4'

config interface 'lan'
	option device 'switch.1'
	...
```

utils.lua should then check lan_vlan's device first, or fallback to lan's device

anygw.lua should configure it's macvlan on top of the device `switch.1` if present
or using the most common 'br-lan'
added then a dynamic retrieval of the right device from `network.lan.device`

tested on:
- dlink,dsl-2750b-b1 (bmips/bcm6328) dsa with switch.1
- tp-link,td-w8968-v3 (bmips/bcm6318) dsa with switch.1
- cudy,wr3000s-v1 (mediatek/filogic) dsa with br-lan

Author: a-gave

packages/lime-proto-anygw/files/usr/lib/lua/lime/proto/anygw.lua

@@ -32,7 +32,7 @@ function anygw.configure(args)
 	local anygw_ipv4 = ipv4:minhost()
 	anygw_ipv6:prefix(64) -- SLAAC only works with a /64, per RFC
 	anygw_ipv4:prefix(ipv4:prefix())
-	local baseIfname = "br-lan"
+	local baseIfname = config.uci:get('network', 'lan', 'device')
 	local argsDev = { macaddr = anygw_mac }
 	local argsIf = {
 		proto = "static",

packages/lime-system/files/usr/lib/lua/lime/utils.lua

@@ -635,7 +635,11 @@ function utils.find_bridge_cfgid(bridge_name)
 end
 
 function utils.find_br_lan()
-    return utils.find_bridge_cfgid("br-lan")
+    local dev = config.uci:get('network', 'lan_vlan', 'device')
+    if dev == nil then
+        dev = config.uci:get('network', 'lan', 'device')
+    end
+    return utils.find_bridge_cfgid(dev)
 end
 
 return utils