Add support for hammerhead style sysfs rgb led controls

While in general I'm a fan of DRY, given that we still repeat the color multiple times as well as the last path element (e.g. `blink_delay_on`) below, I think it might be better to just write out the full paths below (this might allow for easier grepping in source, e.g. "led:rgb_red" would match, but doesn't at the moment.