ar71xx: Fix LED definitions for the DRAGINO2 board

This patch fixes LED definitions for the DRAGINO2 board.

1. It renames the Router/USB led to System, as it is now marked "SYS" on the board.
2. It gives control of the LAN and WAN leds and some other GPIOs to Linux.
3. It fixes the active_low property for the LAN and WAN leds.
4. It sets up WLAN, LAN and WAN leds in the UCI defaults.
5. It allows usage of the System led by the diag.sh script, so it will be used to indicate boot and failsafe status.

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

SVN-Revision: 42897