shell: safe print from different threads

Added display text management to shell_fprintf function.
Now it can be used from diffrent threads with not risk that
displayed lines will overlay.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>
Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
8 files changed