logging: Add autostart option to LOG_BACKEND_DEFINE

Extended macro to accept flag indicating if given backend must be
initialized and enabled when log subsystem starts. Typically, simple
backends will have autostart flag set. More complex may require
explicit enabling (e.g. shell over BLE can only be enabled when
BLE connection is established).

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