dfu/mcuboot: use flash_map instead of flash API

Patch introduces flash_map subsystem to operate on flash
footprint instead of direct operation using flash_driver API.
Flash area ID is used in API instead of direct flash-bank-offsets.
Changes allows to support operation on the partition in any flash
device.

flash_map was not available when this subsystem was introduced.

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