flashfiles.mk: Use du -b to get apparent size

[hao-yao]

On disks using ZFS, du can't get correct size of efi_images_tmp, which caused "Disk full" error. Using apparent size can fix it.

[sysopenci]

Expected syntax:
Commit message one-line summary

  Extra paragraphs can be added here.

  Longer description that explains the problem / solution
  or whatever that needs to be said

  It can contain
    multiple lines
    multiple paragraphs
    blank lines
    references

  But each paragraph must be followed by a blank line

  However, you need to keep the 'Tracked-On', 'Change-Id',
  'Signed-off-by', ..., fields in the **lastest** paragraph of
  the commit message.

  Change-Id: I9e58f79c7faf7f0edb9792e8b5322ab161331141
  Tracked-On: https://jira.server.ext/browse/PROJECTA-123
  Signed-off-by: Developer Name <developer.name@intel.com>