susbys/dfu/img_util: refined ERASE PROGRESSIVELY implementation

Moved MCUboot trailer's status erase ahead any write operation,
which is step which helps with addition of support for devices
don't require explicit pager erase.
For these kind of devices flattening of mcuboot image status in
the trailer was introduced.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>