led: enable LEDST_BLINK state unconditionally

Changes:
 * enable LEDST_BLINK state unconditionally
 * function led_set_period() becomes available when CONFIG_LED_BLINK
   is disabled. This makes led code simpler.
 * fix cmd/led.c to work properly when LEDST_BLINK present, but
   CONFIG_LED_BLINK is disabled

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Simon Glass <sjg@chromium.org>