openthread: Add possibility to initialize OpenThread in POST_KERNEL

The new CONFIG_OPENTHREAD_SYS_INIT Kconfig option allows OpenThread
to be automatically initialised during the Zephyr POST_KERNEL
initialisation stage.

If Zephyr's L2 layer OpenThread implementation is enabled, the
IEEE802.15.4 shim layer initialises OpenThread in the POST_KERNEL
phase. However, since Openthread may work without Zephyr's L2
layer, in this case, no object can initialise it automatically.
This new Kconfig option may help start OpenThread automatically
if the L2 Layer is disabled.

Signed-off-by: Arkadiusz Balys <arkadiusz.balys@nordicsemi.no>