shell: Allow calling shell_fprintf from various contexts

Extended shell to allow command to indicate that shell should
halt not accepting any input until termination sequence is
received (CTRL+C) or shell_command_exit() is called. While shell
is in that state it is allowed to print to shell from any thread
context.

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