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>
(cherry picked from commit a117093679)

scripts/clean-package.sh

@@ -14,7 +14,7 @@ cat "$1" | (
 		[ -f "$entry" ] && rm -f $entry
 	done
 )
-cat "$1" | (
+sort -r "$1" | (
 	cd "$2"
 	while read entry; do
 		[ -n "$entry" ] || break