scarlet: blink charging LED in debug mode
scarlet: blink charging LED in debug mode

The blinking LED indicates that debug mode is active.

BUG=b:69981636
TEST=Press VolUp + VolDown for 10s to enter debug mode
  => charging LED blinks
  => stops blinking after user action or timeout

  test different cases:
  - fully charged battery, connected to power
  - fully charged battery, not connected to power
  - partially discharged battery, connected to power
  - partially discharged battery, not connected to power
BRANCH=none

Change-Id: If7ca06aaf5d1fb4c3df4dfb9e63ca6425002b880
Signed-off-by: Matthias Kaehlcke <df22eae001bf82a113742cff559755b8fc65d45a@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/818496
Reviewed-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec