disk: dos: add some defines for the hardcoded numbers

Add some handy defines for some hardcoded magic numbers related to
extended partition handling.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
diff --git a/disk/part_dos.c b/disk/part_dos.c
index ef706fb..20d35dc 100644
--- a/disk/part_dos.c
+++ b/disk/part_dos.c
@@ -42,9 +42,9 @@
 
 static inline int is_extended(int part_type)
 {
-    return (part_type == 0x5 ||
-	    part_type == 0xf ||
-	    part_type == 0x85);
+    return (part_type == DOS_PART_TYPE_EXTENDED ||
+	    part_type == DOS_PART_TYPE_EXTENDED_LBA ||
+	    part_type == DOS_PART_TYPE_EXTENDED_LINUX);
 }
 
 static int get_bootable(dos_partition_t *p)