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)

Change-Id: I1955261de0f6323518b214e2731ef4879c3304e0
(cherry picked from commit d0412222)