kernel: init: provide access to kernel startup state

Device initialization may require use of generic services such as
starting up power rails, some of which may be controlled by GPIOs on
an external controller that can't be used until full kernel services
are available.  Generic services can check k_is_in_isr() and mediate
their behavior that way, but currently have no way to determine that
the kernel is not available.

Provide a function that indicates whether initialization is still in
pre-kernel stages where no kernel services are available.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>