vold: Check partitions 1-4 instead of 0-3

In mmc_bootstrap_mmcblk, bootstrap partitions 1-4 instead of 0-3, since
that's how the kernel labels them.  Additionally, use the NDOSPART constant
from diskmbr.h instead of hardcoding 4 when scanning partitions.

Signed-off-by: San Mehat <san@google.com>