[CLI] Better Support of Multi-Slot Firmware Updates

[SZenglein]

Reference: https://docs.zephyrproject.org/latest/services/device_mgmt/smp_groups/smp_group_1.html

We support specifying a slot when sending a firmware update, but currently this slot is not used on the next boot with the default zephyr behaviour..

The command to set the "confirm" and "test" flags (see zephyr doc) needs to be implemented in the CLI. The messages are already defined in the library, so it should be a matter of adding the app subcommand and testing it.