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.
diff --git a/vold/blkdev.c b/vold/blkdev.c
index 1482a1a..0a15a73 100644
--- a/vold/blkdev.c
+++ b/vold/blkdev.c
@@ -112,7 +112,7 @@
             goto out;
         }
 
-        for (i = 0; i < 4; i++) {
+        for (i = 0; i < NDOSPART; i++) {
             struct dos_partition part;
 
             dos_partition_dec(block + DOSPARTOFF + i * sizeof(struct dos_partition), &part);
@@ -134,7 +134,7 @@
         struct dos_partition part;
         int part_no = blk->minor -1;
 
-        if (part_no < 4) {
+        if (part_no < NDOSPART) {
             dos_partition_dec(block + DOSPARTOFF + part_no * sizeof(struct dos_partition), &part);
             blk->part_type = part.dp_typ;
         } else {