shell: fix synchronization in log immediate mode

When LOG_MODE_IMMEDIATE is used, logs processed by the shell
log backend may be intertwined with messages printed by
shell commands running on the shell thread.

It is because the shell uses a mutex while the shell log
backend uses the IRQ lock for synchronization. Switch the
latter to use the mutex as well whenever it's possible.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>