cmd: Kconfig: LED command depends on LED subsystems

Without CONFIG_LED, we get

cmd/built-in.o: In function `show_led_state':
cmd/led.c:40: undefined reference to `led_get_state'
cmd/built-in.o: In function `do_led':
cmd/led.c:99: undefined reference to `led_get_by_label'
cmd/led.c:108: undefined reference to `led_set_state'

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>