Shell log backend was using k_fifo to enqueue log messages.
It was using field in log message that was used for same
purpose in log_core before passing message to backends.
However, this method supported only single shell as
other shell was corruption the fifo because field was
reused.
Modified shell log backend to use k_msgq for pending
messages.
Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
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>
