base-files: introduce commonly used helper functions for setting leds

Currently, most platforms define such helpers in their own diag.sh
implementation with almost identical code. By factoring out the
common ground it's possible to simplify maintainability and
homogenize the haptics over multiple platforms (so far as is
reasonably practicable, in a next step).

[juhosg:
 - fix led_set_attr parameters in led_timer
 - add led_morse and status_led_set_morse helpers
 - add status_led_blink_{preinit,failsafe} helpers]

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35648 3c298f89-4303-0410-b956-a3cf2f4a3e73