ext/lib/mgmt/mcumgr: align zephyr port to changes in dfu sybsy'es

Changes in flash_map API makes flash_area structure proper
interface for point the image area instead of direct flash-bank-offsets.

This patch align code to changed APIa and allows to support operation
on the partition in any flash device.

Signed-off-by: Findlay Feng <i@fengch.me>