led: shutdown values for simple PWM leds

[Laikulo]

This adds support for configuring an on-shutdown for PWM-style LEDs. This is useful for end-users who have a status or stack lamp for indicating machine status, either as a single RGB, or as multiple LEDs.

For example:

[board_pins ...]
mcu: ...
aliases:
  alias_stack_red: ...
  alias_stack_grn: ...
  alias_stack_blu: ...

[led stack]
red_pin:   stack_red
green_pin: stack_grn
blue_pin:  stack_blu

# Initial State: Waiting
initial_RED:   1
initial_GREEN: 1
initial_BLUE:  0

# Shutdown State: Fault
shutdown_RED:   1
shutdown_GREEN: 0
shutdown_BLUE:  0

## Other sections (incl. macros) define behavior during normal operation.