wic: use truncate utility to create sparse files

Used truncate instead of dd to create wic images for the
following reasons:
 - dd doesn't preserve sparseness
 - truncate syntax is much more clear
 - dd requires additional calculations of the image size
   in blocks
 - the way dd was used in the code is not always correct.
   In some cases it was writing one block to the file which makes
   it not 100% sparse.

[YOCTO #9099]

(From OE-Core rev: d2d0d18dfd3922411d856b98ab6ba5d64c9c1c9f)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/utils/fs_related.py

@@ -71,14 +71,8 @@ class DiskImage(Disk):
     def create(self):
         if self.device is not None:
             return
-
-        blocks = self.size / 1024
-        if self.size - blocks * 1024:
-            blocks += 1
-
-        # create disk image
-        dd_cmd = "dd if=/dev/zero of=%s bs=1024 seek=%d count=1" % \
-            (self.image_file, blocks)
-        exec_cmd(dd_cmd)
+        # create sparse disk image
+        cmd = "truncate %s -s %s" % (self.image_file, self.size)
+        exec_cmd(cmd)
 
         self.device = self.image_file