purge-locales: fix handling of X11 locale.dir

The /usr/share/X11/locale/locale.dir file is needed by libX11.
Removing it breaks locale support in X11. However, make removes
not only directories but also all files, which are not listed
in the BR2_ENABLE_LOCALE_WHITELIST.

This re-creates locale.dir database file where needed.

Signed-off-by: Valentine Barshak <gvaxon@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Arnout: use a separate loop, and add some explanatory comments]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>