mediatek: GL-MT6000: Change LED colors

mediatek: GL-MT6000: Change LED colors

Fine tuning PR: openwrt/openwrt#14355 Ref: 5a82bb909bf1
("mediatek: GL-MT6000: Add missing LED state definitions")

As the only LED is using white in the stock firmware when the device is
running and blue for the bootloader I suggest following changes:
 - Using blue for the BL and preinit+failsafe
 - White for normal operation (like the original FW) and sysupgrade

With this changes it's clear by looking to the LED in which operation
mode the device is and a possible BL stuck can be seen easily.

Tested with [GL-MT6000](https://openwrt.org/toh/gl.inet/gl-mt6000).

Signed-off-by: Thomas Schröder <tschroeder_github@outlook.com>
Tested-by: Hannu Nyman <hannu.nyman@iki.fi>
(cherry picked from commit 4d7bac1dcaae019fb4992bfb74348ef12c02daa4)