scripts: ipkg-make-index.sh: dereference symbolic links

scripts: ipkg-make-index.sh: dereference symbolic links

Use `stat -L` instead of `ls -l` to follow symbolic links when obtaining
the file size of .ipk archives.

Without this change, the size of the symlink, not the size of the target
file is encoded in the package index file.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit ece5cab743f9df6c9655d6117e92fda110292173)
Fixes: e6af9c017b0c ("opkg: bump to version 2019-06-14")
[ rmilecki: this has to be backported due to the recent opkg update and
  cb6640381808 ("libopkg: check for file size mismatches") to fix false
  "opkg_install_pkg: Package size mismatch" errors ]
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>