base-files/leds: add setting the LED color via uci

base-files/leds: add setting the LED color via uci

Add the possibility that colored LEDs can also be configured via the uci.

config led 'led1'
	option name '<name>'
	option sysfs '<path>'
	option trigger 'default-on'
	option default '1'
-->	option color_{$color} '<0-255>'

The supported names of the variable "${color}" for the selected LED can be
queried in the file with the name 'multi_index'.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>