logging: Add option to block in thread context

Added CONIFG_LOG_BLOCK_IN_THREAD option to block until buffer for
log message is available. When log message is called in the thread
and there is no buffer available in the pool, thread will block with
configurable timeout (CONFIG_LOG_BLOCK_IN_THREAD_TIMEOUT_MS). If
buffer cannot be allocated by that time, message will be dropped.

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