LuCI now supports the updated UCI syntax for bridges that requires:
1. device section for L2
2. interface section for L3
Check for legacy syntax usage and offser user a migration to allow
changing network config.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit bca76a767316a689d59dfd4974dcb7bfb04db1e8)