build: fix STAGING_DIR cleaning for packages

build: fix STAGING_DIR cleaning for packages

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>