write_sparse_image could be useful for non-fastboot users.
For ex a platform, without usb-device/fastboot support, could
get sparse images over tftp and write using the mmc command.
Or non-android systems could also leverage the sparse format.

Towards that, this patch removes anything fastboot specific from
the write_sparse_image implementation. Which includes making the
function return integer as error code and calls for fastboot logging
via an optional callback function 'mssg'.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>