We've already had the 'open_channel' and 'close_channel' commands,
which were sent to (and acknowledged by) the card. However,
those commands didn't affect the pySim-shell state, i.e. all
communication would still happen through the default channel '0'.
With this patch we introduce a 'switch_channel' command, using which
the user can determine which of the (previously opened) logical channels
shall be used by pySim-shell.
Change-Id: Ia76eb45c4925882ae6866e50b64d9610bd4d546d
Closes: OS#6230