boot: boot_serial: Fix issue with CBOR and setting image state

Fixes an issue whereby when canonical mode for ZCBOR was enabled,
the state variables were not increased to handle the backup
states, and a bug whereby only secondary slots were checked for
status when setting image state, whilst generally this is the
intended outcome, a user should also be able to mark a primary
image as confirmed too for other modes such as direct-xip

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>