disk/part: check bootable flag for DOS partitions

Determine which partitions are bootable/active. In the partition listing,
print "Boot" for partitions with the bootable/active flag set.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
diff --git a/include/part.h b/include/part.h
index e1478f4..447f69d 100644
--- a/include/part.h
+++ b/include/part.h
@@ -93,6 +93,7 @@
 	ulong	blksz;		/* block size in bytes			*/
 	uchar	name[32];	/* partition name			*/
 	uchar	type[32];	/* string type description		*/
+	int	bootable;	/* Active/Bootable flag is set		*/
 } disk_partition_t;
 
 /* Misc _get_dev functions */