usb: device_next: allow selection of alternative USB device context

When we enable USBD shell support with samples like cdc_acm or mass,
there would be another USB device context besides the one provided
by the shell. This patch introduces a new command to select an
alternate context to be used by USBD shell commands.

Also fix the type where "add" command should be "remove".

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>