logging: Postpone log processing when no backends enabled

In initial state of the system it is possible that no backends are
yet enabled and log processing is triggered. In that case logs were
discarded. A flag has been added to the log core to do processing
only if at least one backend is enabled. It is applicable only to
initial phase. If backends are disabled in application lifetime
logs will be discarded.

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