This fixes two issues with cleaning package files from STAGING_DIR:
* CleanStaging currently can only remove files and not directories. This
changes CleanStaging to use clean-package.sh, which does remove
directories.
* Because of the way directories are ordered in the staging files list,
clean-package.sh currently tries (and fails) to remove parent
directories before removing subdirectories. This changes
clean-package.sh to process the staging files list in reverse, so that
subdirectories are removed first.
Signed-off-by: Jeffery To <jeffery.to@gmail.com>