libs/uci: implement get_bool() to retrive uci variables as boolean value

libs/uci/luasrc/model/uci.lua

@@ -135,6 +135,16 @@ function Cursor.tset(self, config, section, values)
 	return stat
 end
 
+--- Get a boolean option and return it's value as true or false.
+-- @param config	UCI config
+-- @param section	UCI section name
+-- @param option	UCI option
+-- @return			Boolean
+function Cursor.get_bool(self, ...)
+	local val = self:get(...)
+	return ( val == "1" or val == "true" or val == "yes" or val == "on" )
+end
+
 --- Get an option or list and return values as table.
 -- @param config	UCI config
 -- @param section	UCI section name