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>
96a610b736