releasetools: Use du -b

* When $OUT is on a compressed disk,
  `du -k -s` might not return the correct sizes.
* Adding `-b` gets us close to the actual size.

toybox commit: https://github.com/landley/toybox/pull/177

Test: m, builds and boots.
Devices: crosshatch blueline bonito sargo (anything with dynamic partitions)

tools/releasetools/build_image.py

@@ -57,7 +57,7 @@ def GetDiskUsage(path):
   Returns:
     The number of bytes based on a 1K block_size.
   """
-  cmd = ["du", "-k", "-s", path]
+  cmd = ["du", "-b", "-k", "-s", path]
   output = common.RunAndCheckOutput(cmd, verbose=False)
   return int(output.split()[0]) * 1024