mmc: Separate "mmc swrite" from fastboot

Introduce CONFIG_IMAGE_SPARSE and CONFIG_CMD_MMC_SWRITE so the "mmc
swrite" command is separated from the fastboot code.

Move image-sparse from common to lib so it's clear it's library code.

Rename CONFIG_FASTBOOT_FLASH_FILLBUF_SIZE to CONFIG_IMAGE_SPARSE_FILLBUF_SIZE
and migrate it to Kconfig.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>