base-files: Fix config_generate when there are no switch VLANs or ports configured in board.json.

The json_select call fails when there are no roles or ports objects in board.json. "json_select .." must not be executed after failing.

This fixes for example LEDs not being set up in /etc/config/system.

Signed-off-by: Vittorio Gambaletta <openwrt@vittgam.net>