Handle that `IMAGE_OFFSET` can be set to something that contain spaces.

With 'IMAGE_OFFSET=$$((0x1000 + 0x1000))' we had in the command line when doing
'flash'

    --offset $((0x1000 --offset + --offset 0x1000))

With the change we correctly have

    --offset $((0x1000 + 0x1000))