add uci led support

- /etc/config/system can hold led info which is applied at boot
- adds /sbin/led.sh to set/clear leds via a name defined in /etc/config/system



git-svn-id: svn://svn.openwrt.org/openwrt/trunk@12104 3c298f89-4303-0410-b956-a3cf2f4a3e73