Added dummy backend which can be enabled with Kconfig. By default it is
disabled because it needs the same amount of memory as other phisical
backends. It shall be use only for commands testing purposes.
Improved shell_execute_cmd function, now it clears command context
before new command will be executed.
Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>
Jakub Rzeszutko