west.yml: Update mcumgr revision

Commits affecting Zephyr that are included with the new revision:
    12b496e Add IMG_MGMT_FRUGAL_LIST configuration option;
    18c628d cborattr: fixes missing error handling;
    2e385b6 zephyr: Fix usage of zephyr_img_mgmt_flash_area_id.

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