fastboot: Rename public fb_ functions to fastboot_

Rename fb_mmc_flash_write/fb_mmc_erase/fb_nand_flash_write/fb_nand_erase to
fastboot_... as they form a public interface

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>