mgmt/mcumgr: Replace use of flash_area_erase with flash_area_flatten

The commit replaces flash_area_erase with flash_area_flatten.
The function is used in to places:
 1) in image management commands IMG_MGMT_ID_UPLOAD
    and IMG_MGMT_ID_ERASE: to erase an image in secondary slot
    or to scramble trailer part of image, which could be misunderstood
    by MCUboot as valid image operation request;
 2) in command ZEPHYR_MGMT_GRP_BASIC_CMD_ERASE_STORAGE to
    erase/scramble data partition.

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