luci-app-mwan3: add config for mwan3 mask

Add global config option to edit the mwan3 firewall mask.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

applications/luci-app-mwan3/luasrc/model/cbi/mwan/globalsconfig.lua

@@ -29,4 +29,12 @@ for _, net in ipairs(net:get_networks()) do
 end
 n.rmempty = false
 
+mask = s:option(
+	Value,
+	"mmx_mask",
+	translate("Firewall mask"),
+	translate("Enter value in hex, starting with <code>0x</code>"))
+mask.datatype = "hex(4)"
+mask.default = "0xff00"
+
 return m