shell: Fix immediate logging case

When logging is using immediate mode then logging messages
can be processed from any context, including interrupt context.
z_shell_fprintf was asserting in that case since it allowed to
be called from interrupt context only when logging was in panic
mode. However, shell works in the same way when logging is in
immediate mode as in panic mode.

Renamed internal shell flag from panic_mode to sync_mode. Flag
is also set when shell log backend is started in synchronous
mode (immediate logging) which prevents assertion.

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