Currently, a symbolic link whose target is a directory will not be
removed when cleaning packages from STAGING_DIR.

In the first cleaning pass in scripts/clean-package.sh, the -f test for
a directory symlink returns false (because the link target is a
directory) and so the symlink is not removed.

In the second pass, the -d test returns true for a directory symlink,
but the symlink is not removed by rmdir because rmdir only removes
(real) directories.

This updates clean-package.sh to remove all non-directories (including
symbolic links) in the first pass.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>