mmcblk mount

mount vfat , -> read the first 512 bytes from the whole block device.
and find the file system flags.

blkdev.c:
/*
* Open the disk partition table
*/
devpath = blkdev_get_devpath(blk->disk)


dos_partition_dec(block + DOSPARTOFF + ((part_no>4)?part_no-blk->disk->minor:part_no) * sizeof(struct dos_partition), &part);