cmd: avb: Support A/B slots

Add optional parameter to 'avb verify' sub-command, so that user is able
to specify which slot to use, in case when user's partitions are
slotted. If that parameter is omitted, the behavior of 'avb verify' will
be the same as before, so user API is content.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Acked-by: Igor Opaniuk <igor.opaniuk@gmail.com>