mgmt/MCUmgr/grp/img: Switch to using img_mgmt_get_next_boot_slot

The commit modifies image list command operations to use
img_mgmt_get_next_boot_slot instead of directly relying of
MCUboot flags.
The function is now used, also, by img_mgmt_slot_in_use to
figure out whether queried slot is in use.
The commit introduces two new Kconfig options
 MCUMGR_GRP_IMG_ALLOW_CONFIRM_NON_ACTIVE_IMAGE_SECONDARY
 MCUMGR_GRP_IMG_ALLOW_CONFIRM_NON_ACTIVE_IMAGE_ANY
that allow users to enable confirming non-active images slots.
The MCUMGR_GRP_IMG_ALLOW_CONFIRM_NON_ACTIVE_IMAGE_SECONDARY is y
by default to keep original behavior of logic that accidentally
allowed confirming secondary slot.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>