mmc: support writing sparse images

Provide an alternate path for sparse-images to be
written to MMC. For example, via tftp on platforms
that don't support fastboot protocol. Or when an
image is to written at some offset, rather than the
start of a partition.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
[trini: Guard with CONFIG_FASTBOOT_FLASH tests, use LBAF for lbaint_t
        printing]
Signed-off-by: Tom Rini <trini@konsulko.com>