logging: Add option for prolonged backend initialization

Extended logging backend API with log_backend_is_ready call which
returns 0 is backend is ready. Logging core will make sure that
all autostarted backends are ready before they are enabled.

This option allows to handle backends which are not yet ready
after init function is called (e.g. usb backend that is not plugged
in). If this is the only backend in the system, logging processing
will not start util first backend is ready.

Function for checking readiness is optional and when backend has
no such function it is assumed that backend is ready after
initialization function returns which makes this feature backward
compatible.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>